Revert "Revert "Implement call combiner""

This reverts commit bf19961d0a49b43cb528392efeb4880eeebb9b5e.

60 files changed, 1255 insertions, 867 deletions

diff --git a/BUILD b/BUILD
index 67e727bd52..b8062e4297 100644
--- a/BUILD
+++ b/BUILD
@@ -577,6 +577,7 @@ grpc_cc_library(
         "src/core/lib/http/format_request.c",
         "src/core/lib/http/httpcli.c",
         "src/core/lib/http/parser.c",
+        "src/core/lib/iomgr/call_combiner.c",
         "src/core/lib/iomgr/closure.c",
         "src/core/lib/iomgr/combiner.c",
         "src/core/lib/iomgr/endpoint.c",
@@ -709,6 +710,7 @@ grpc_cc_library(
         "src/core/lib/http/format_request.h",
         "src/core/lib/http/httpcli.h",
         "src/core/lib/http/parser.h",
+        "src/core/lib/iomgr/call_combiner.h",
         "src/core/lib/iomgr/closure.h",
         "src/core/lib/iomgr/combiner.h",
         "src/core/lib/iomgr/endpoint.h",
diff --git a/CMakeLists.txt b/CMakeLists.txt
index efc33c0a6c..6f374ff947 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -967,6 +967,7 @@ add_library(grpc
   src/core/lib/http/format_request.c
   src/core/lib/http/httpcli.c
   src/core/lib/http/parser.c
+  src/core/lib/iomgr/call_combiner.c
   src/core/lib/iomgr/closure.c
   src/core/lib/iomgr/combiner.c
   src/core/lib/iomgr/endpoint.c
@@ -1318,6 +1319,7 @@ add_library(grpc_cronet
   src/core/lib/http/format_request.c
   src/core/lib/http/httpcli.c
   src/core/lib/http/parser.c
+  src/core/lib/iomgr/call_combiner.c
   src/core/lib/iomgr/closure.c
   src/core/lib/iomgr/combiner.c
   src/core/lib/iomgr/endpoint.c
@@ -1637,6 +1639,7 @@ add_library(grpc_test_util
   src/core/lib/http/format_request.c
   src/core/lib/http/httpcli.c
   src/core/lib/http/parser.c
+  src/core/lib/iomgr/call_combiner.c
   src/core/lib/iomgr/closure.c
   src/core/lib/iomgr/combiner.c
   src/core/lib/iomgr/endpoint.c
@@ -1900,6 +1903,7 @@ add_library(grpc_test_util_unsecure
   src/core/lib/http/format_request.c
   src/core/lib/http/httpcli.c
   src/core/lib/http/parser.c
+  src/core/lib/iomgr/call_combiner.c
   src/core/lib/iomgr/closure.c
   src/core/lib/iomgr/combiner.c
   src/core/lib/iomgr/endpoint.c
@@ -2149,6 +2153,7 @@ add_library(grpc_unsecure
   src/core/lib/http/format_request.c
   src/core/lib/http/httpcli.c
   src/core/lib/http/parser.c
+  src/core/lib/iomgr/call_combiner.c
   src/core/lib/iomgr/closure.c
   src/core/lib/iomgr/combiner.c
   src/core/lib/iomgr/endpoint.c
@@ -2850,6 +2855,7 @@ add_library(grpc++_cronet
   src/core/lib/http/format_request.c
   src/core/lib/http/httpcli.c
   src/core/lib/http/parser.c
+  src/core/lib/iomgr/call_combiner.c
   src/core/lib/iomgr/closure.c
   src/core/lib/iomgr/combiner.c
   src/core/lib/iomgr/endpoint.c
diff --git a/Makefile b/Makefile
index 4fd534df09..c0ca5cdc3b 100644
--- a/Makefile
+++ b/Makefile
@@ -2912,6 +2912,7 @@ LIBGRPC_SRC = \
     src/core/lib/http/format_request.c \
     src/core/lib/http/httpcli.c \
     src/core/lib/http/parser.c \
+    src/core/lib/iomgr/call_combiner.c \
     src/core/lib/iomgr/closure.c \
     src/core/lib/iomgr/combiner.c \
     src/core/lib/iomgr/endpoint.c \
@@ -3261,6 +3262,7 @@ LIBGRPC_CRONET_SRC = \
     src/core/lib/http/format_request.c \
     src/core/lib/http/httpcli.c \
     src/core/lib/http/parser.c \
+    src/core/lib/iomgr/call_combiner.c \
     src/core/lib/iomgr/closure.c \
     src/core/lib/iomgr/combiner.c \
     src/core/lib/iomgr/endpoint.c \
@@ -3577,6 +3579,7 @@ LIBGRPC_TEST_UTIL_SRC = \
     src/core/lib/http/format_request.c \
     src/core/lib/http/httpcli.c \
     src/core/lib/http/parser.c \
+    src/core/lib/iomgr/call_combiner.c \
     src/core/lib/iomgr/closure.c \
     src/core/lib/iomgr/combiner.c \
     src/core/lib/iomgr/endpoint.c \
@@ -3829,6 +3832,7 @@ LIBGRPC_TEST_UTIL_UNSECURE_SRC = \
     src/core/lib/http/format_request.c \
     src/core/lib/http/httpcli.c \
     src/core/lib/http/parser.c \
+    src/core/lib/iomgr/call_combiner.c \
     src/core/lib/iomgr/closure.c \
     src/core/lib/iomgr/combiner.c \
     src/core/lib/iomgr/endpoint.c \
@@ -4054,6 +4058,7 @@ LIBGRPC_UNSECURE_SRC = \
     src/core/lib/http/format_request.c \
     src/core/lib/http/httpcli.c \
     src/core/lib/http/parser.c \
+    src/core/lib/iomgr/call_combiner.c \
     src/core/lib/iomgr/closure.c \
     src/core/lib/iomgr/combiner.c \
     src/core/lib/iomgr/endpoint.c \
@@ -4738,6 +4743,7 @@ LIBGRPC++_CRONET_SRC = \
     src/core/lib/http/format_request.c \
     src/core/lib/http/httpcli.c \
     src/core/lib/http/parser.c \
+    src/core/lib/iomgr/call_combiner.c \
     src/core/lib/iomgr/closure.c \
     src/core/lib/iomgr/combiner.c \
     src/core/lib/iomgr/endpoint.c \
diff --git a/binding.gyp b/binding.gyp
index 946edd8139..ef2f3d0223 100644
--- a/binding.gyp
+++ b/binding.gyp
@@ -672,6 +672,7 @@
         'src/core/lib/http/format_request.c',
         'src/core/lib/http/httpcli.c',
         'src/core/lib/http/parser.c',
+        'src/core/lib/iomgr/call_combiner.c',
         'src/core/lib/iomgr/closure.c',
         'src/core/lib/iomgr/combiner.c',
         'src/core/lib/iomgr/endpoint.c',
diff --git a/build.yaml b/build.yaml
index a43ad6ae31..5264b4adb8 100644
--- a/build.yaml
+++ b/build.yaml
@@ -200,6 +200,7 @@ filegroups:
   - src/core/lib/http/format_request.c
   - src/core/lib/http/httpcli.c
   - src/core/lib/http/parser.c
+  - src/core/lib/iomgr/call_combiner.c
   - src/core/lib/iomgr/closure.c
   - src/core/lib/iomgr/combiner.c
   - src/core/lib/iomgr/endpoint.c
@@ -352,6 +353,7 @@ filegroups:
   - src/core/lib/http/format_request.h
   - src/core/lib/http/httpcli.h
   - src/core/lib/http/parser.h
+  - src/core/lib/iomgr/call_combiner.h
   - src/core/lib/iomgr/closure.h
   - src/core/lib/iomgr/combiner.h
   - src/core/lib/iomgr/endpoint.h
diff --git a/config.m4 b/config.m4
index d9d25cb006..b7af75d949 100644
--- a/config.m4
+++ b/config.m4
@@ -101,6 +101,7 @@ if test "$PHP_GRPC" != "no"; then
     src/core/lib/http/format_request.c \
     src/core/lib/http/httpcli.c \
     src/core/lib/http/parser.c \
+    src/core/lib/iomgr/call_combiner.c \
     src/core/lib/iomgr/closure.c \
     src/core/lib/iomgr/combiner.c \
     src/core/lib/iomgr/endpoint.c \
diff --git a/config.w32 b/config.w32
index 640115e861..1a64d23ac7 100644
--- a/config.w32
+++ b/config.w32
@@ -78,6 +78,7 @@ if (PHP_GRPC != "no") {
     "src\\core\\lib\\http\\format_request.c " +
     "src\\core\\lib\\http\\httpcli.c " +
     "src\\core\\lib\\http\\parser.c " +
+    "src\\core\\lib\\iomgr\\call_combiner.c " +
     "src\\core\\lib\\iomgr\\closure.c " +
     "src\\core\\lib\\iomgr\\combiner.c " +
     "src\\core\\lib\\iomgr\\endpoint.c " +
diff --git a/doc/environment_variables.md b/doc/environment_variables.md
index a2cde92736..0dae0543ff 100644
--- a/doc/environment_variables.md
+++ b/doc/environment_variables.md
@@ -39,6 +39,7 @@ some configuration as environment variables that can be set.
   gRPC C core is processing requests via debug logs. Available tracers include:
   - api - traces api calls to the C core
   - bdp_estimator - traces behavior of bdp estimation logic
+  - call_combiner - traces call combiner state
   - call_error - traces the possible errors contributing to final call status
   - channel - traces operations on the C core channel stack
   - client_channel - traces client channel activity, including resolver
diff --git a/gRPC-Core.podspec b/gRPC-Core.podspec
index e82f917bdb..c76b520808 100644
--- a/gRPC-Core.podspec
+++ b/gRPC-Core.podspec
@@ -334,6 +334,7 @@ Pod::Spec.new do |s|
                       'src/core/lib/http/format_request.h',
                       'src/core/lib/http/httpcli.h',
                       'src/core/lib/http/parser.h',
+                      'src/core/lib/iomgr/call_combiner.h',
                       'src/core/lib/iomgr/closure.h',
                       'src/core/lib/iomgr/combiner.h',
                       'src/core/lib/iomgr/endpoint.h',
@@ -484,6 +485,7 @@ Pod::Spec.new do |s|
                       'src/core/lib/http/format_request.c',
                       'src/core/lib/http/httpcli.c',
                       'src/core/lib/http/parser.c',
+                      'src/core/lib/iomgr/call_combiner.c',
                       'src/core/lib/iomgr/closure.c',
                       'src/core/lib/iomgr/combiner.c',
                       'src/core/lib/iomgr/endpoint.c',
@@ -831,6 +833,7 @@ Pod::Spec.new do |s|
                               'src/core/lib/http/format_request.h',
                               'src/core/lib/http/httpcli.h',
                               'src/core/lib/http/parser.h',
+                              'src/core/lib/iomgr/call_combiner.h',
                               'src/core/lib/iomgr/closure.h',
                               'src/core/lib/iomgr/combiner.h',
                               'src/core/lib/iomgr/endpoint.h',
diff --git a/grpc.gemspec b/grpc.gemspec
index 2b41f13469..61376b8165 100644
--- a/grpc.gemspec
+++ b/grpc.gemspec
@@ -266,6 +266,7 @@ Gem::Specification.new do |s|
   s.files += %w( src/core/lib/http/format_request.h )
   s.files += %w( src/core/lib/http/httpcli.h )
   s.files += %w( src/core/lib/http/parser.h )
+  s.files += %w( src/core/lib/iomgr/call_combiner.h )
   s.files += %w( src/core/lib/iomgr/closure.h )
   s.files += %w( src/core/lib/iomgr/combiner.h )
   s.files += %w( src/core/lib/iomgr/endpoint.h )
@@ -420,6 +421,7 @@ Gem::Specification.new do |s|
   s.files += %w( src/core/lib/http/format_request.c )
   s.files += %w( src/core/lib/http/httpcli.c )
   s.files += %w( src/core/lib/http/parser.c )
+  s.files += %w( src/core/lib/iomgr/call_combiner.c )
   s.files += %w( src/core/lib/iomgr/closure.c )
   s.files += %w( src/core/lib/iomgr/combiner.c )
   s.files += %w( src/core/lib/iomgr/endpoint.c )
diff --git a/grpc.gyp b/grpc.gyp
index 40938a4564..5c4510a211 100644
--- a/grpc.gyp
+++ b/grpc.gyp
@@ -238,6 +238,7 @@
         'src/core/lib/http/format_request.c',
         'src/core/lib/http/httpcli.c',
         'src/core/lib/http/parser.c',
+        'src/core/lib/iomgr/call_combiner.c',
         'src/core/lib/iomgr/closure.c',
         'src/core/lib/iomgr/combiner.c',
         'src/core/lib/iomgr/endpoint.c',
@@ -538,6 +539,7 @@
         'src/core/lib/http/format_request.c',
         'src/core/lib/http/httpcli.c',
         'src/core/lib/http/parser.c',
+        'src/core/lib/iomgr/call_combiner.c',
         'src/core/lib/iomgr/closure.c',
         'src/core/lib/iomgr/combiner.c',
         'src/core/lib/iomgr/endpoint.c',
@@ -743,6 +745,7 @@
         'src/core/lib/http/format_request.c',
         'src/core/lib/http/httpcli.c',
         'src/core/lib/http/parser.c',
+        'src/core/lib/iomgr/call_combiner.c',
         'src/core/lib/iomgr/closure.c',
         'src/core/lib/iomgr/combiner.c',
         'src/core/lib/iomgr/endpoint.c',
@@ -933,6 +936,7 @@
         'src/core/lib/http/format_request.c',
         'src/core/lib/http/httpcli.c',
         'src/core/lib/http/parser.c',
+        'src/core/lib/iomgr/call_combiner.c',
         'src/core/lib/iomgr/closure.c',
         'src/core/lib/iomgr/combiner.c',
         'src/core/lib/iomgr/endpoint.c',
diff --git a/include/grpc/impl/codegen/atm.h b/include/grpc/impl/codegen/atm.h
index 2cfd22ab63..764bee5272 100644
--- a/include/grpc/impl/codegen/atm.h
+++ b/include/grpc/impl/codegen/atm.h
@@ -46,6 +46,7 @@
 
    // Atomically return *p, with acquire semantics.
    gpr_atm gpr_atm_acq_load(gpr_atm *p);
+   gpr_atm gpr_atm_no_barrier_load(gpr_atm *p);
 
    // Atomically set *p = value, with release semantics.
    void gpr_atm_rel_store(gpr_atm *p, gpr_atm value);
diff --git a/package.xml b/package.xml
index 1cca4fbddd..f588fc5a20 100644
--- a/package.xml
+++ b/package.xml
@@ -276,6 +276,7 @@
     <file baseinstalldir="/" name="src/core/lib/http/format_request.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/http/httpcli.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/http/parser.h" role="src" />
+    <file baseinstalldir="/" name="src/core/lib/iomgr/call_combiner.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/iomgr/closure.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/iomgr/combiner.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/iomgr/endpoint.h" role="src" />
@@ -430,6 +431,7 @@
     <file baseinstalldir="/" name="src/core/lib/http/format_request.c" role="src" />
     <file baseinstalldir="/" name="src/core/lib/http/httpcli.c" role="src" />
     <file baseinstalldir="/" name="src/core/lib/http/parser.c" role="src" />
+    <file baseinstalldir="/" name="src/core/lib/iomgr/call_combiner.c" role="src" />
     <file baseinstalldir="/" name="src/core/lib/iomgr/closure.c" role="src" />
     <file baseinstalldir="/" name="src/core/lib/iomgr/combiner.c" role="src" />
     <file baseinstalldir="/" name="src/core/lib/iomgr/endpoint.c" role="src" />
diff --git a/src/core/ext/census/grpc_filter.c b/src/core/ext/census/grpc_filter.c
index 13fe2e6b1c..3e10f61154 100644
--- a/src/core/ext/census/grpc_filter.c
+++ b/src/core/ext/census/grpc_filter.c
@@ -179,7 +179,6 @@ const grpc_channel_filter grpc_client_census_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "census-client"};
 
@@ -193,6 +192,5 @@ const grpc_channel_filter grpc_server_census_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "census-server"};
diff --git a/src/core/ext/filters/client_channel/client_channel.c b/src/core/ext/filters/client_channel/client_channel.c
index 58e31d7b45..e6822ce801 100644
--- a/src/core/ext/filters/client_channel/client_channel.c
+++ b/src/core/ext/filters/client_channel/client_channel.c
@@ -796,7 +796,8 @@ static void cc_destroy_channel_elem(grpc_exec_ctx *exec_ctx,
 //   send_message
 //   recv_trailing_metadata
 //   send_trailing_metadata
-#define MAX_WAITING_BATCHES 6
+// We also add room for a single cancel_stream batch.
+#define MAX_WAITING_BATCHES 7
 
 /** Call data.  Holds a pointer to grpc_subchannel_call and the
     associated machinery to create such a pointer.
@@ -808,23 +809,25 @@ typedef struct client_channel_call_data {
   // The code in deadline_filter.c requires this to be the first field.
   // TODO(roth): This is slightly sub-optimal in that grpc_deadline_state
   // and this struct both independently store a pointer to the call
-  // stack and each has its own mutex.  If/when we have time, find a way
-  // to avoid this without breaking the grpc_deadline_state abstraction.
+  // combiner.  If/when we have time, find a way to avoid this without
+  // breaking the grpc_deadline_state abstraction.
   grpc_deadline_state deadline_state;
 
   grpc_slice path;  // Request path.
   gpr_timespec call_start_time;
   gpr_timespec deadline;
+  gpr_arena *arena;
+  grpc_call_combiner *call_combiner;
+
   grpc_server_retry_throttle_data *retry_throttle_data;
   method_parameters *method_params;
 
-  /** either 0 for no call, a pointer to a grpc_subchannel_call (if the lowest
-      bit is 0), or a pointer to an error (if the lowest bit is 1) */
-  gpr_atm subchannel_call_or_error;
-  gpr_arena *arena;
+  grpc_subchannel_call *subchannel_call;
+  grpc_error *error;
 
   grpc_lb_policy *lb_policy;  // Holds ref while LB pick is pending.
   grpc_closure lb_pick_closure;
+  grpc_closure cancel_closure;
 
   grpc_connected_subchannel *connected_subchannel;
   grpc_call_context_element subchannel_call_context[GRPC_CONTEXT_COUNT];
@@ -832,10 +835,9 @@ typedef struct client_channel_call_data {
 
   grpc_transport_stream_op_batch *waiting_for_pick_batches[MAX_WAITING_BATCHES];
   size_t waiting_for_pick_batches_count;
+  grpc_closure handle_pending_batch_in_call_combiner[MAX_WAITING_BATCHES];
 
-  grpc_transport_stream_op_batch_payload *initial_metadata_payload;
-
-  grpc_call_stack *owning_call;
+  grpc_transport_stream_op_batch *initial_metadata_batch;
 
   grpc_linked_mdelem lb_token_mdelem;
 
@@ -843,55 +845,42 @@ typedef struct client_channel_call_data {
   grpc_closure *original_on_complete;
 } call_data;
 
-typedef struct {
-  grpc_subchannel_call *subchannel_call;
-  grpc_error *error;
-} call_or_error;
-
-static call_or_error get_call_or_error(call_data *p) {
-  gpr_atm c = gpr_atm_acq_load(&p->subchannel_call_or_error);
-  if (c == 0)
-    return (call_or_error){NULL, NULL};
-  else if (c & 1)
-    return (call_or_error){NULL, (grpc_error *)((c) & ~(gpr_atm)1)};
-  else
-    return (call_or_error){(grpc_subchannel_call *)c, NULL};
+grpc_subchannel_call *grpc_client_channel_get_subchannel_call(
+    grpc_call_element *elem) {
+  call_data *calld = elem->call_data;
+  return calld->subchannel_call;
 }
 
-static bool set_call_or_error(call_data *p, call_or_error coe) {
-  // this should always be under a lock
-  call_or_error existing = get_call_or_error(p);
-  if (existing.error != GRPC_ERROR_NONE) {
-    GRPC_ERROR_UNREF(coe.error);
-    return false;
-  }
-  GPR_ASSERT(existing.subchannel_call == NULL);
-  if (coe.error != GRPC_ERROR_NONE) {
-    GPR_ASSERT(coe.subchannel_call == NULL);
-    gpr_atm_rel_store(&p->subchannel_call_or_error, 1 | (gpr_atm)coe.error);
+// This is called via the call combiner, so access to calld is synchronized.
+static void waiting_for_pick_batches_add(
+    call_data *calld, grpc_transport_stream_op_batch *batch) {
+  if (batch->send_initial_metadata) {
+    GPR_ASSERT(calld->initial_metadata_batch == NULL);
+    calld->initial_metadata_batch = batch;
   } else {
-    GPR_ASSERT(coe.subchannel_call != NULL);
-    gpr_atm_rel_store(&p->subchannel_call_or_error,
-                      (gpr_atm)coe.subchannel_call);
+    GPR_ASSERT(calld->waiting_for_pick_batches_count < MAX_WAITING_BATCHES);
+    calld->waiting_for_pick_batches[calld->waiting_for_pick_batches_count++] =
+        batch;
   }
-  return true;
 }
 
-grpc_subchannel_call *grpc_client_channel_get_subchannel_call(
-    grpc_call_element *call_elem) {
-  return get_call_or_error(call_elem->call_data).subchannel_call;
-}
-
-static void waiting_for_pick_batches_add_locked(
-    call_data *calld, grpc_transport_stream_op_batch *batch) {
-  GPR_ASSERT(calld->waiting_for_pick_batches_count < MAX_WAITING_BATCHES);
-  calld->waiting_for_pick_batches[calld->waiting_for_pick_batches_count++] =
-      batch;
+// This is called via the call combiner, so access to calld is synchronized.
+static void fail_pending_batch_in_call_combiner(grpc_exec_ctx *exec_ctx,
+                                                void *arg, grpc_error *error) {
+  call_data *calld = arg;
+  if (calld->waiting_for_pick_batches_count > 0) {
+    --calld->waiting_for_pick_batches_count;
+    grpc_transport_stream_op_batch_finish_with_failure(
+        exec_ctx,
+        calld->waiting_for_pick_batches[calld->waiting_for_pick_batches_count],
+        GRPC_ERROR_REF(error), calld->call_combiner);
+  }
 }
 
-static void waiting_for_pick_batches_fail_locked(grpc_exec_ctx *exec_ctx,
-                                                 grpc_call_element *elem,
-                                                 grpc_error *error) {
+// This is called via the call combiner, so access to calld is synchronized.
+static void waiting_for_pick_batches_fail(grpc_exec_ctx *exec_ctx,
+                                          grpc_call_element *elem,
+                                          grpc_error *error) {
   call_data *calld = elem->call_data;
   if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
     gpr_log(GPR_DEBUG,
@@ -900,34 +889,60 @@ static void waiting_for_pick_batches_fail_locked(grpc_exec_ctx *exec_ctx,
             grpc_error_string(error));
   }
   for (size_t i = 0; i < calld->waiting_for_pick_batches_count; ++i) {
+    GRPC_CLOSURE_INIT(&calld->handle_pending_batch_in_call_combiner[i],
+                      fail_pending_batch_in_call_combiner, calld,
+                      grpc_schedule_on_exec_ctx);
+    GRPC_CALL_COMBINER_START(exec_ctx, calld->call_combiner,
+                             &calld->handle_pending_batch_in_call_combiner[i],
+                             GRPC_ERROR_REF(error),
+                             "waiting_for_pick_batches_fail");
+  }
+  if (calld->initial_metadata_batch != NULL) {
     grpc_transport_stream_op_batch_finish_with_failure(
-        exec_ctx, calld->waiting_for_pick_batches[i], GRPC_ERROR_REF(error));
+        exec_ctx, calld->initial_metadata_batch, GRPC_ERROR_REF(error),
+        calld->call_combiner);
+  } else {
+    GRPC_CALL_COMBINER_STOP(exec_ctx, calld->call_combiner,
+                            "waiting_for_pick_batches_fail");
   }
-  calld->waiting_for_pick_batches_count = 0;
   GRPC_ERROR_UNREF(error);
 }
 
-static void waiting_for_pick_batches_resume_locked(grpc_exec_ctx *exec_ctx,
-                                                   grpc_call_element *elem) {
-  call_data *calld = elem->call_data;
-  if (calld->waiting_for_pick_batches_count == 0) return;
-  call_or_error coe = get_call_or_error(calld);
-  if (coe.error != GRPC_ERROR_NONE) {
-    waiting_for_pick_batches_fail_locked(exec_ctx, elem,
-                                         GRPC_ERROR_REF(coe.error));
-    return;
+// This is called via the call combiner, so access to calld is synchronized.
+static void run_pending_batch_in_call_combiner(grpc_exec_ctx *exec_ctx,
+                                               void *arg, grpc_error *ignored) {
+  call_data *calld = arg;
+  if (calld->waiting_for_pick_batches_count > 0) {
+    --calld->waiting_for_pick_batches_count;
+    grpc_subchannel_call_process_op(
+        exec_ctx, calld->subchannel_call,
+        calld->waiting_for_pick_batches[calld->waiting_for_pick_batches_count]);
   }
+}
+
+// This is called via the call combiner, so access to calld is synchronized.
+static void waiting_for_pick_batches_resume(grpc_exec_ctx *exec_ctx,
+                                            grpc_call_element *elem) {
+  channel_data *chand = elem->channel_data;
+  call_data *calld = elem->call_data;
   if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
     gpr_log(GPR_DEBUG, "chand=%p calld=%p: sending %" PRIdPTR
                        " pending batches to subchannel_call=%p",
-            elem->channel_data, calld, calld->waiting_for_pick_batches_count,
-            coe.subchannel_call);
+            chand, calld, calld->waiting_for_pick_batches_count,
+            calld->subchannel_call);
   }
   for (size_t i = 0; i < calld->waiting_for_pick_batches_count; ++i) {
-    grpc_subchannel_call_process_op(exec_ctx, coe.subchannel_call,
-                                    calld->waiting_for_pick_batches[i]);
-  }
-  calld->waiting_for_pick_batches_count = 0;
+    GRPC_CLOSURE_INIT(&calld->handle_pending_batch_in_call_combiner[i],
+                      run_pending_batch_in_call_combiner, calld,
+                      grpc_schedule_on_exec_ctx);
+    GRPC_CALL_COMBINER_START(exec_ctx, calld->call_combiner,
+                             &calld->handle_pending_batch_in_call_combiner[i],
+                             GRPC_ERROR_NONE,
+                             "waiting_for_pick_batches_resume");
+  }
+  GPR_ASSERT(calld->initial_metadata_batch != NULL);
+  grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call,
+                                  calld->initial_metadata_batch);
 }
 
 // Applies service config to the call.  Must be invoked once we know
@@ -968,29 +983,28 @@ static void apply_service_config_to_call_locked(grpc_exec_ctx *exec_ctx,
 static void create_subchannel_call_locked(grpc_exec_ctx *exec_ctx,
                                           grpc_call_element *elem,
                                           grpc_error *error) {
+  channel_data *chand = elem->channel_data;
   call_data *calld = elem->call_data;
-  grpc_subchannel_call *subchannel_call = NULL;
   const grpc_connected_subchannel_call_args call_args = {
       .pollent = calld->pollent,
       .path = calld->path,
       .start_time = calld->call_start_time,
       .deadline = calld->deadline,
       .arena = calld->arena,
-      .context = calld->subchannel_call_context};
+      .context = calld->subchannel_call_context,
+      .call_combiner = calld->call_combiner};
   grpc_error *new_error = grpc_connected_subchannel_create_call(
-      exec_ctx, calld->connected_subchannel, &call_args, &subchannel_call);
+      exec_ctx, calld->connected_subchannel, &call_args,
+      &calld->subchannel_call);
   if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
     gpr_log(GPR_DEBUG, "chand=%p calld=%p: create subchannel_call=%p: error=%s",
-            elem->channel_data, calld, subchannel_call,
-            grpc_error_string(new_error));
+            chand, calld, calld->subchannel_call, grpc_error_string(new_error));
   }
-  GPR_ASSERT(set_call_or_error(
-      calld, (call_or_error){.subchannel_call = subchannel_call}));
   if (new_error != GRPC_ERROR_NONE) {
     new_error = grpc_error_add_child(new_error, error);
-    waiting_for_pick_batches_fail_locked(exec_ctx, elem, new_error);
+    waiting_for_pick_batches_fail(exec_ctx, elem, new_error);
   } else {
-    waiting_for_pick_batches_resume_locked(exec_ctx, elem);
+    waiting_for_pick_batches_resume(exec_ctx, elem);
   }
   GRPC_ERROR_UNREF(error);
 }
@@ -1002,60 +1016,27 @@ static void subchannel_ready_locked(grpc_exec_ctx *exec_ctx,
   channel_data *chand = elem->channel_data;
   grpc_polling_entity_del_from_pollset_set(exec_ctx, calld->pollent,
                                            chand->interested_parties);
-  call_or_error coe = get_call_or_error(calld);
   if (calld->connected_subchannel == NULL) {
     // Failed to create subchannel.
-    grpc_error *failure =
-        error == GRPC_ERROR_NONE
-            ? GRPC_ERROR_CREATE_FROM_STATIC_STRING(
-                  "Call dropped by load balancing policy")
-            : GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
-                  "Failed to create subchannel", &error, 1);
+    GRPC_ERROR_UNREF(calld->error);
+    calld->error = error == GRPC_ERROR_NONE
+                       ? GRPC_ERROR_CREATE_FROM_STATIC_STRING(
+                             "Call dropped by load balancing policy")
+                       : GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
+                             "Failed to create subchannel", &error, 1);
     if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
       gpr_log(GPR_DEBUG,
               "chand=%p calld=%p: failed to create subchannel: error=%s", chand,
-              calld, grpc_error_string(failure));
-    }
-    set_call_or_error(calld, (call_or_error){.error = GRPC_ERROR_REF(failure)});
-    waiting_for_pick_batches_fail_locked(exec_ctx, elem, failure);
-  } else if (coe.error != GRPC_ERROR_NONE) {
-    /* already cancelled before subchannel became ready */
-    grpc_error *child_errors[] = {error, coe.error};
-    grpc_error *cancellation_error =
-        GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
-            "Cancelled before creating subchannel", child_errors,
-            GPR_ARRAY_SIZE(child_errors));
-    /* if due to deadline, attach the deadline exceeded status to the error */
-    if (gpr_time_cmp(calld->deadline, gpr_now(GPR_CLOCK_MONOTONIC)) < 0) {
-      cancellation_error =
-          grpc_error_set_int(cancellation_error, GRPC_ERROR_INT_GRPC_STATUS,
-                             GRPC_STATUS_DEADLINE_EXCEEDED);
+              calld, grpc_error_string(calld->error));
     }
-    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-      gpr_log(GPR_DEBUG,
-              "chand=%p calld=%p: cancelled before subchannel became ready: %s",
-              chand, calld, grpc_error_string(cancellation_error));
-    }
-    waiting_for_pick_batches_fail_locked(exec_ctx, elem, cancellation_error);
+    waiting_for_pick_batches_fail(exec_ctx, elem, GRPC_ERROR_REF(calld->error));
   } else {
     /* Create call on subchannel. */
     create_subchannel_call_locked(exec_ctx, elem, GRPC_ERROR_REF(error));
   }
-  GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call, "pick_subchannel");
   GRPC_ERROR_UNREF(error);
 }
 
-static char *cc_get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  call_data *calld = elem->call_data;
-  grpc_subchannel_call *subchannel_call =
-      get_call_or_error(calld).subchannel_call;
-  if (subchannel_call == NULL) {
-    return NULL;
-  } else {
-    return grpc_subchannel_call_get_peer(exec_ctx, subchannel_call);
-  }
-}
-
 /** Return true if subchannel is available immediately (in which case
     subchannel_ready_locked() should not be called), or false otherwise (in
     which case subchannel_ready_locked() should be called when the subchannel
@@ -1069,6 +1050,44 @@ typedef struct {
   grpc_closure closure;
 } pick_after_resolver_result_args;
 
+// Note: This runs under the client_channel combiner, but will NOT be
+// holding the call combiner.
+static void pick_after_resolver_result_cancel_locked(grpc_exec_ctx *exec_ctx,
+                                                     void *arg,
+                                                     grpc_error *error) {
+  grpc_call_element *elem = arg;
+  channel_data *chand = elem->channel_data;
+  call_data *calld = elem->call_data;
+  // If we don't yet have a resolver result, then a closure for
+  // pick_after_resolver_result_done_locked() will have been added to
+  // chand->waiting_for_resolver_result_closures, and it may not be invoked
+  // until after this call has been destroyed.  We mark the operation as
+  // cancelled, so that when pick_after_resolver_result_done_locked()
+  // is called, it will be a no-op.  We also immediately invoke
+  // subchannel_ready_locked() to propagate the error back to the caller.
+  for (grpc_closure *closure = chand->waiting_for_resolver_result_closures.head;
+       closure != NULL; closure = closure->next_data.next) {
+    pick_after_resolver_result_args *args = closure->cb_arg;
+    if (!args->cancelled && args->elem == elem) {
+      if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
+        gpr_log(GPR_DEBUG,
+                "chand=%p calld=%p: "
+                "cancelling pick waiting for resolver result",
+                chand, calld);
+      }
+      args->cancelled = true;
+      // Note: Although we are not in the call combiner here, we are
+      // basically stealing the call combiner from the pending pick, so
+      // it's safe to call subchannel_ready_locked() here -- we are
+      // essentially calling it here instead of calling it in
+      // pick_after_resolver_result_done_locked().
+      subchannel_ready_locked(exec_ctx, elem,
+                              GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
+                                  "Pick cancelled", &error, 1));
+    }
+  }
+}
+
 static void pick_after_resolver_result_done_locked(grpc_exec_ctx *exec_ctx,
                                                    void *arg,
                                                    grpc_error *error) {
@@ -1079,21 +1098,24 @@ static void pick_after_resolver_result_done_locked(grpc_exec_ctx *exec_ctx,
       gpr_log(GPR_DEBUG, "call cancelled before resolver result");
     }
   } else {
-    channel_data *chand = args->elem->channel_data;
-    call_data *calld = args->elem->call_data;
+    grpc_call_element *elem = args->elem;
+    channel_data *chand = elem->channel_data;
+    call_data *calld = elem->call_data;
+    grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner,
+                                            NULL);
     if (error != GRPC_ERROR_NONE) {
       if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
         gpr_log(GPR_DEBUG, "chand=%p calld=%p: resolver failed to return data",
                 chand, calld);
       }
-      subchannel_ready_locked(exec_ctx, args->elem, GRPC_ERROR_REF(error));
+      subchannel_ready_locked(exec_ctx, elem, GRPC_ERROR_REF(error));
     } else {
       if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
         gpr_log(GPR_DEBUG, "chand=%p calld=%p: resolver returned, doing pick",
                 chand, calld);
       }
-      if (pick_subchannel_locked(exec_ctx, args->elem)) {
-        subchannel_ready_locked(exec_ctx, args->elem, GRPC_ERROR_NONE);
+      if (pick_subchannel_locked(exec_ctx, elem)) {
+        subchannel_ready_locked(exec_ctx, elem, GRPC_ERROR_NONE);
       }
     }
   }
@@ -1116,41 +1138,33 @@ static void pick_after_resolver_result_start_locked(grpc_exec_ctx *exec_ctx,
                     args, grpc_combiner_scheduler(chand->combiner));
   grpc_closure_list_append(&chand->waiting_for_resolver_result_closures,
                            &args->closure, GRPC_ERROR_NONE);
+  grpc_call_combiner_set_notify_on_cancel(
+      exec_ctx, calld->call_combiner,
+      GRPC_CLOSURE_INIT(&calld->cancel_closure,
+                        pick_after_resolver_result_cancel_locked, elem,
+                        grpc_combiner_scheduler(chand->combiner)));
 }
 
-static void pick_after_resolver_result_cancel_locked(grpc_exec_ctx *exec_ctx,
-                                                     grpc_call_element *elem,
-                                                     grpc_error *error) {
+// Note: This runs under the client_channel combiner, but will NOT be
+// holding the call combiner.
+static void pick_callback_cancel_locked(grpc_exec_ctx *exec_ctx, void *arg,
+                                        grpc_error *error) {
+  grpc_call_element *elem = arg;
   channel_data *chand = elem->channel_data;
   call_data *calld = elem->call_data;
-  // If we don't yet have a resolver result, then a closure for
-  // pick_after_resolver_result_done_locked() will have been added to
-  // chand->waiting_for_resolver_result_closures, and it may not be invoked
-  // until after this call has been destroyed.  We mark the operation as
-  // cancelled, so that when pick_after_resolver_result_done_locked()
-  // is called, it will be a no-op.  We also immediately invoke
-  // subchannel_ready_locked() to propagate the error back to the caller.
-  for (grpc_closure *closure = chand->waiting_for_resolver_result_closures.head;
-       closure != NULL; closure = closure->next_data.next) {
-    pick_after_resolver_result_args *args = closure->cb_arg;
-    if (!args->cancelled && args->elem == elem) {
-      if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-        gpr_log(GPR_DEBUG,
-                "chand=%p calld=%p: "
-                "cancelling pick waiting for resolver result",
-                chand, calld);
-      }
-      args->cancelled = true;
-      subchannel_ready_locked(exec_ctx, elem,
-                              GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
-                                  "Pick cancelled", &error, 1));
+  if (calld->lb_policy != NULL) {
+    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
+      gpr_log(GPR_DEBUG, "chand=%p calld=%p: cancelling pick from LB policy %p",
+              chand, calld, calld->lb_policy);
     }
+    grpc_lb_policy_cancel_pick_locked(exec_ctx, calld->lb_policy,
+                                      &calld->connected_subchannel,
+                                      GRPC_ERROR_REF(error));
   }
-  GRPC_ERROR_UNREF(error);
 }
 
 // Callback invoked by grpc_lb_policy_pick_locked() for async picks.
-// Unrefs the LB policy after invoking subchannel_ready_locked().
+// Unrefs the LB policy and invokes subchannel_ready_locked().
 static void pick_callback_done_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                       grpc_error *error) {
   grpc_call_element *elem = arg;
@@ -1160,6 +1174,7 @@ static void pick_callback_done_locked(grpc_exec_ctx *exec_ctx, void *arg,
     gpr_log(GPR_DEBUG, "chand=%p calld=%p: pick completed asynchronously",
             chand, calld);
   }
+  grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner, NULL);
   GPR_ASSERT(calld->lb_policy != NULL);
   GRPC_LB_POLICY_UNREF(exec_ctx, calld->lb_policy, "pick_subchannel");
   calld->lb_policy = NULL;
@@ -1194,24 +1209,15 @@ static bool pick_callback_start_locked(grpc_exec_ctx *exec_ctx,
     }
     GRPC_LB_POLICY_UNREF(exec_ctx, calld->lb_policy, "pick_subchannel");
     calld->lb_policy = NULL;
+  } else {
+    grpc_call_combiner_set_notify_on_cancel(
+        exec_ctx, calld->call_combiner,
+        GRPC_CLOSURE_INIT(&calld->cancel_closure, pick_callback_cancel_locked,
+                          elem, grpc_combiner_scheduler(chand->combiner)));
   }
   return pick_done;
 }
 
-static void pick_callback_cancel_locked(grpc_exec_ctx *exec_ctx,
-                                        grpc_call_element *elem,
-                                        grpc_error *error) {
-  channel_data *chand = elem->channel_data;
-  call_data *calld = elem->call_data;
-  GPR_ASSERT(calld->lb_policy != NULL);
-  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-    gpr_log(GPR_DEBUG, "chand=%p calld=%p: cancelling pick from LB policy %p",
-            chand, calld, calld->lb_policy);
-  }
-  grpc_lb_policy_cancel_pick_locked(exec_ctx, calld->lb_policy,
-                                    &calld->connected_subchannel, error);
-}
-
 static bool pick_subchannel_locked(grpc_exec_ctx *exec_ctx,
                                    grpc_call_element *elem) {
   GPR_TIMER_BEGIN("pick_subchannel", 0);
@@ -1224,7 +1230,7 @@ static bool pick_subchannel_locked(grpc_exec_ctx *exec_ctx,
     // Otherwise, if the service config specified a value for this
     // method, use that.
     uint32_t initial_metadata_flags =
-        calld->initial_metadata_payload->send_initial_metadata
+        calld->initial_metadata_batch->payload->send_initial_metadata
             .send_initial_metadata_flags;
     const bool wait_for_ready_set_from_api =
         initial_metadata_flags &
@@ -1241,7 +1247,7 @@ static bool pick_subchannel_locked(grpc_exec_ctx *exec_ctx,
       }
     }
     const grpc_lb_policy_pick_args inputs = {
-        calld->initial_metadata_payload->send_initial_metadata
+        calld->initial_metadata_batch->payload->send_initial_metadata
             .send_initial_metadata,
         initial_metadata_flags, &calld->lb_token_mdelem};
     pick_done = pick_callback_start_locked(exec_ctx, elem, &inputs);
@@ -1258,91 +1264,33 @@ static bool pick_subchannel_locked(grpc_exec_ctx *exec_ctx,
   return pick_done;
 }
 
-static void start_transport_stream_op_batch_locked(grpc_exec_ctx *exec_ctx,
-                                                   void *arg,
-                                                   grpc_error *error_ignored) {
-  GPR_TIMER_BEGIN("start_transport_stream_op_batch_locked", 0);
-  grpc_transport_stream_op_batch *batch = arg;
-  grpc_call_element *elem = batch->handler_private.extra_arg;
-  call_data *calld = elem->call_data;
-  channel_data *chand = elem->channel_data;
-  /* need to recheck that another thread hasn't set the call */
-  call_or_error coe = get_call_or_error(calld);
-  if (coe.error != GRPC_ERROR_NONE) {
-    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-      gpr_log(GPR_DEBUG, "chand=%p calld=%p: failing batch with error: %s",
-              chand, calld, grpc_error_string(coe.error));
-    }
-    grpc_transport_stream_op_batch_finish_with_failure(
-        exec_ctx, batch, GRPC_ERROR_REF(coe.error));
-    goto done;
-  }
-  if (coe.subchannel_call != NULL) {
-    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-      gpr_log(GPR_DEBUG,
-              "chand=%p calld=%p: sending batch to subchannel_call=%p", chand,
-              calld, coe.subchannel_call);
-    }
-    grpc_subchannel_call_process_op(exec_ctx, coe.subchannel_call, batch);
-    goto done;
-  }
-  // Add to waiting-for-pick list.  If we succeed in getting a
-  // subchannel call below, we'll handle this batch (along with any
-  // other waiting batches) in waiting_for_pick_batches_resume_locked().
-  waiting_for_pick_batches_add_locked(calld, batch);
-  // If this is a cancellation, cancel the pending pick (if any) and
-  // fail any pending batches.
-  if (batch->cancel_stream) {
-    grpc_error *error = batch->payload->cancel_stream.cancel_error;
-    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-      gpr_log(GPR_DEBUG, "chand=%p calld=%p: recording cancel_error=%s", chand,
-              calld, grpc_error_string(error));
-    }
-    /* Stash a copy of cancel_error in our call data, so that we can use
-       it for subsequent operations.  This ensures that if the call is
-       cancelled before any batches are passed down (e.g., if the deadline
-       is in the past when the call starts), we can return the right
-       error to the caller when the first batch does get passed down. */
-    set_call_or_error(calld, (call_or_error){.error = GRPC_ERROR_REF(error)});
-    if (calld->lb_policy != NULL) {
-      pick_callback_cancel_locked(exec_ctx, elem, GRPC_ERROR_REF(error));
+static void start_pick_locked(grpc_exec_ctx *exec_ctx, void *arg,
+                              grpc_error *error_ignored) {
+  GPR_TIMER_BEGIN("start_pick_locked", 0);
+  grpc_call_element *elem = (grpc_call_element *)arg;
+  call_data *calld = (call_data *)elem->call_data;
+  channel_data *chand = (channel_data *)elem->channel_data;
+  GPR_ASSERT(calld->connected_subchannel == NULL);
+  if (pick_subchannel_locked(exec_ctx, elem)) {
+    // Pick was returned synchronously.
+    if (calld->connected_subchannel == NULL) {
+      GRPC_ERROR_UNREF(calld->error);
+      calld->error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
+          "Call dropped by load balancing policy");
+      waiting_for_pick_batches_fail(exec_ctx, elem,
+                                    GRPC_ERROR_REF(calld->error));
     } else {
-      pick_after_resolver_result_cancel_locked(exec_ctx, elem,
-                                               GRPC_ERROR_REF(error));
-    }
-    waiting_for_pick_batches_fail_locked(exec_ctx, elem, GRPC_ERROR_REF(error));
-    goto done;
-  }
-  /* if we don't have a subchannel, try to get one */
-  if (batch->send_initial_metadata) {
-    GPR_ASSERT(calld->connected_subchannel == NULL);
-    calld->initial_metadata_payload = batch->payload;
-    GRPC_CALL_STACK_REF(calld->owning_call, "pick_subchannel");
-    /* If a subchannel is not available immediately, the polling entity from
-       call_data should be provided to channel_data's interested_parties, so
-       that IO of the lb_policy and resolver could be done under it. */
-    if (pick_subchannel_locked(exec_ctx, elem)) {
-      // Pick was returned synchronously.
-      GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call, "pick_subchannel");
-      if (calld->connected_subchannel == NULL) {
-        grpc_error *error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
-            "Call dropped by load balancing policy");
-        set_call_or_error(calld,
-                          (call_or_error){.error = GRPC_ERROR_REF(error)});
-        waiting_for_pick_batches_fail_locked(exec_ctx, elem, error);
-      } else {
-        // Create subchannel call.
-        create_subchannel_call_locked(exec_ctx, elem, GRPC_ERROR_NONE);
-      }
-    } else {
-      grpc_polling_entity_add_to_pollset_set(exec_ctx, calld->pollent,
-                                             chand->interested_parties);
+      // Create subchannel call.
+      create_subchannel_call_locked(exec_ctx, elem, GRPC_ERROR_NONE);
     }
+  } else {
+    // Pick will be done asynchronously.  Add the call's polling entity to
+    // the channel's interested_parties, so that I/O for the resolver
+    // and LB policy can be done under it.
+    grpc_polling_entity_add_to_pollset_set(exec_ctx, calld->pollent,
+                                           chand->interested_parties);
   }
-done:
-  GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call,
-                        "start_transport_stream_op_batch");
-  GPR_TIMER_END("start_transport_stream_op_batch_locked", 0);
+  GPR_TIMER_END("start_pick_locked", 0);
 }
 
 static void on_complete(grpc_exec_ctx *exec_ctx, void *arg, grpc_error *error) {
@@ -1365,27 +1313,49 @@ static void on_complete(grpc_exec_ctx *exec_ctx, void *arg, grpc_error *error) {
                    GRPC_ERROR_REF(error));
 }
 
-/* The logic here is fairly complicated, due to (a) the fact that we
-   need to handle the case where we receive the send op before the
-   initial metadata op, and (b) the need for efficiency, especially in
-   the streaming case.
-
-   We use double-checked locking to initially see if initialization has been
-   performed. If it has not, we acquire the combiner and perform initialization.
-   If it has, we proceed on the fast path. */
 static void cc_start_transport_stream_op_batch(
     grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
     grpc_transport_stream_op_batch *batch) {
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
-  if (GRPC_TRACER_ON(grpc_client_channel_trace) ||
-      GRPC_TRACER_ON(grpc_trace_channel)) {
-    grpc_call_log_op(GPR_INFO, elem, batch);
-  }
   if (chand->deadline_checking_enabled) {
     grpc_deadline_state_client_start_transport_stream_op_batch(exec_ctx, elem,
                                                                batch);
   }
+  GPR_TIMER_BEGIN("cc_start_transport_stream_op_batch", 0);
+  // If we've previously been cancelled, immediately fail any new batches.
+  if (calld->error != GRPC_ERROR_NONE) {
+    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
+      gpr_log(GPR_DEBUG, "chand=%p calld=%p: failing batch with error: %s",
+              chand, calld, grpc_error_string(calld->error));
+    }
+    grpc_transport_stream_op_batch_finish_with_failure(
+        exec_ctx, batch, GRPC_ERROR_REF(calld->error), calld->call_combiner);
+    goto done;
+  }
+  if (batch->cancel_stream) {
+    // Stash a copy of cancel_error in our call data, so that we can use
+    // it for subsequent operations.  This ensures that if the call is
+    // cancelled before any batches are passed down (e.g., if the deadline
+    // is in the past when the call starts), we can return the right
+    // error to the caller when the first batch does get passed down.
+    GRPC_ERROR_UNREF(calld->error);
+    calld->error = GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error);
+    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
+      gpr_log(GPR_DEBUG, "chand=%p calld=%p: recording cancel_error=%s", chand,
+              calld, grpc_error_string(calld->error));
+    }
+    // If we have a subchannel call, send the cancellation batch down.
+    // Otherwise, fail all pending batches.
+    if (calld->subchannel_call != NULL) {
+      grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call, batch);
+    } else {
+      waiting_for_pick_batches_add(calld, batch);
+      waiting_for_pick_batches_fail(exec_ctx, elem,
+                                    GRPC_ERROR_REF(calld->error));
+    }
+    goto done;
+  }
   // Intercept on_complete for recv_trailing_metadata so that we can
   // check retry throttle status.
   if (batch->recv_trailing_metadata) {
@@ -1395,38 +1365,43 @@ static void cc_start_transport_stream_op_batch(
                       grpc_schedule_on_exec_ctx);
     batch->on_complete = &calld->on_complete;
   }
-  /* try to (atomically) get the call */
-  call_or_error coe = get_call_or_error(calld);
-  GPR_TIMER_BEGIN("cc_start_transport_stream_op_batch", 0);
-  if (coe.error != GRPC_ERROR_NONE) {
+  // Check if we've already gotten a subchannel call.
+  // Note that once we have completed the pick, we do not need to enter
+  // the channel combiner, which is more efficient (especially for
+  // streaming calls).
+  if (calld->subchannel_call != NULL) {
     if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-      gpr_log(GPR_DEBUG, "chand=%p calld=%p: failing batch with error: %s",
-              chand, calld, grpc_error_string(coe.error));
+      gpr_log(GPR_DEBUG,
+              "chand=%p calld=%p: sending batch to subchannel_call=%p", chand,
+              calld, calld->subchannel_call);
     }
-    grpc_transport_stream_op_batch_finish_with_failure(
-        exec_ctx, batch, GRPC_ERROR_REF(coe.error));
+    grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call, batch);
     goto done;
   }
-  if (coe.subchannel_call != NULL) {
+  // We do not yet have a subchannel call.
+  // Add the batch to the waiting-for-pick list.
+  waiting_for_pick_batches_add(calld, batch);
+  // For batches containing a send_initial_metadata op, enter the channel
+  // combiner to start a pick.
+  if (batch->send_initial_metadata) {
+    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
+      gpr_log(GPR_DEBUG, "chand=%p calld=%p: entering combiner", chand, calld);
+    }
+    GRPC_CLOSURE_SCHED(
+        exec_ctx,
+        GRPC_CLOSURE_INIT(&batch->handler_private.closure, start_pick_locked,
+                          elem, grpc_combiner_scheduler(chand->combiner)),
+        GRPC_ERROR_NONE);
+  } else {
+    // For all other batches, release the call combiner.
     if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
       gpr_log(GPR_DEBUG,
-              "chand=%p calld=%p: sending batch to subchannel_call=%p", chand,
-              calld, coe.subchannel_call);
+              "chand=%p calld=%p: saved batch, yeilding call combiner", chand,
+              calld);
     }
-    grpc_subchannel_call_process_op(exec_ctx, coe.subchannel_call, batch);
-    goto done;
-  }
-  /* we failed; lock and figure out what to do */
-  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
-    gpr_log(GPR_DEBUG, "chand=%p calld=%p: entering combiner", chand, calld);
+    GRPC_CALL_COMBINER_STOP(exec_ctx, calld->call_combiner,
+                            "batch does not include send_initial_metadata");
   }
-  GRPC_CALL_STACK_REF(calld->owning_call, "start_transport_stream_op_batch");
-  batch->handler_private.extra_arg = elem;
-  GRPC_CLOSURE_SCHED(
-      exec_ctx, GRPC_CLOSURE_INIT(&batch->handler_private.closure,
-                                  start_transport_stream_op_batch_locked, batch,
-                                  grpc_combiner_scheduler(chand->combiner)),
-      GRPC_ERROR_NONE);
 done:
   GPR_TIMER_END("cc_start_transport_stream_op_batch", 0);
 }
@@ -1441,10 +1416,11 @@ static grpc_error *cc_init_call_elem(grpc_exec_ctx *exec_ctx,
   calld->path = grpc_slice_ref_internal(args->path);
   calld->call_start_time = args->start_time;
   calld->deadline = gpr_convert_clock_type(args->deadline, GPR_CLOCK_MONOTONIC);
-  calld->owning_call = args->call_stack;
   calld->arena = args->arena;
+  calld->call_combiner = args->call_combiner;
   if (chand->deadline_checking_enabled) {
-    grpc_deadline_state_init(exec_ctx, elem, args->call_stack, calld->deadline);
+    grpc_deadline_state_init(exec_ctx, elem, args->call_stack,
+                             args->call_combiner, calld->deadline);
   }
   return GRPC_ERROR_NONE;
 }
@@ -1463,13 +1439,12 @@ static void cc_destroy_call_elem(grpc_exec_ctx *exec_ctx,
   if (calld->method_params != NULL) {
     method_parameters_unref(calld->method_params);
   }
-  call_or_error coe = get_call_or_error(calld);
-  GRPC_ERROR_UNREF(coe.error);
-  if (coe.subchannel_call != NULL) {
-    grpc_subchannel_call_set_cleanup_closure(coe.subchannel_call,
+  GRPC_ERROR_UNREF(calld->error);
+  if (calld->subchannel_call != NULL) {
+    grpc_subchannel_call_set_cleanup_closure(calld->subchannel_call,
                                              then_schedule_closure);
     then_schedule_closure = NULL;
-    GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, coe.subchannel_call,
+    GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, calld->subchannel_call,
                                "client_channel_destroy_call");
   }
   GPR_ASSERT(calld->lb_policy == NULL);
@@ -1508,7 +1483,6 @@ const grpc_channel_filter grpc_client_channel_filter = {
     sizeof(channel_data),
     cc_init_channel_elem,
     cc_destroy_channel_elem,
-    cc_get_peer,
     cc_get_channel_info,
     "client-channel",
 };
diff --git a/src/core/ext/filters/client_channel/lb_policy/grpclb/client_load_reporting_filter.c b/src/core/ext/filters/client_channel/lb_policy/grpclb/client_load_reporting_filter.c
index 568bb2ba8d..299f26b4de 100644
--- a/src/core/ext/filters/client_channel/lb_policy/grpclb/client_load_reporting_filter.c
+++ b/src/core/ext/filters/client_channel/lb_policy/grpclb/client_load_reporting_filter.c
@@ -132,6 +132,5 @@ const grpc_channel_filter grpc_client_load_reporting_filter = {
     0,  // sizeof(channel_data)
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "client_load_reporting"};
diff --git a/src/core/ext/filters/client_channel/subchannel.c b/src/core/ext/filters/client_channel/subchannel.c
index 5788819331..5cc8be7628 100644
--- a/src/core/ext/filters/client_channel/subchannel.c
+++ b/src/core/ext/filters/client_channel/subchannel.c
@@ -724,13 +724,6 @@ void grpc_subchannel_call_unref(grpc_exec_ctx *exec_ctx,
   GRPC_CALL_STACK_UNREF(exec_ctx, SUBCHANNEL_CALL_TO_CALL_STACK(c), REF_REASON);
 }
 
-char *grpc_subchannel_call_get_peer(grpc_exec_ctx *exec_ctx,
-                                    grpc_subchannel_call *call) {
-  grpc_call_stack *call_stack = SUBCHANNEL_CALL_TO_CALL_STACK(call);
-  grpc_call_element *top_elem = grpc_call_stack_element(call_stack, 0);
-  return top_elem->filter->get_peer(exec_ctx, top_elem);
-}
-
 void grpc_subchannel_call_process_op(grpc_exec_ctx *exec_ctx,
                                      grpc_subchannel_call *call,
                                      grpc_transport_stream_op_batch *op) {
@@ -760,13 +753,15 @@ grpc_error *grpc_connected_subchannel_create_call(
       args->arena, sizeof(grpc_subchannel_call) + chanstk->call_stack_size);
   grpc_call_stack *callstk = SUBCHANNEL_CALL_TO_CALL_STACK(*call);
   (*call)->connection = GRPC_CONNECTED_SUBCHANNEL_REF(con, "subchannel_call");
-  const grpc_call_element_args call_args = {.call_stack = callstk,
-                                            .server_transport_data = NULL,
-                                            .context = args->context,
-                                            .path = args->path,
-                                            .start_time = args->start_time,
-                                            .deadline = args->deadline,
-                                            .arena = args->arena};
+  const grpc_call_element_args call_args = {
+      .call_stack = callstk,
+      .server_transport_data = NULL,
+      .context = args->context,
+      .path = args->path,
+      .start_time = args->start_time,
+      .deadline = args->deadline,
+      .arena = args->arena,
+      .call_combiner = args->call_combiner};
   grpc_error *error = grpc_call_stack_init(
       exec_ctx, chanstk, 1, subchannel_call_destroy, *call, &call_args);
   if (error != GRPC_ERROR_NONE) {
diff --git a/src/core/ext/filters/client_channel/subchannel.h b/src/core/ext/filters/client_channel/subchannel.h
index 6d2abb04df..51d712f6a7 100644
--- a/src/core/ext/filters/client_channel/subchannel.h
+++ b/src/core/ext/filters/client_channel/subchannel.h
@@ -106,6 +106,7 @@ typedef struct {
   gpr_timespec deadline;
   gpr_arena *arena;
   grpc_call_context_element *context;
+  grpc_call_combiner *call_combiner;
 } grpc_connected_subchannel_call_args;
 
 grpc_error *grpc_connected_subchannel_create_call(
@@ -150,10 +151,6 @@ void grpc_subchannel_call_process_op(grpc_exec_ctx *exec_ctx,
                                      grpc_subchannel_call *subchannel_call,
                                      grpc_transport_stream_op_batch *op);
 
-/** continue querying for peer */
-char *grpc_subchannel_call_get_peer(grpc_exec_ctx *exec_ctx,
-                                    grpc_subchannel_call *subchannel_call);
-
 /** Must be called once per call. Sets the 'then_schedule_closure' argument for
     call stack destruction. */
 void grpc_subchannel_call_set_cleanup_closure(
diff --git a/src/core/ext/filters/deadline/deadline_filter.c b/src/core/ext/filters/deadline/deadline_filter.c
index 6789903c95..565b0679dc 100644
--- a/src/core/ext/filters/deadline/deadline_filter.c
+++ b/src/core/ext/filters/deadline/deadline_filter.c
@@ -34,22 +34,56 @@
 // grpc_deadline_state
 //
 
+// The on_complete callback used when sending a cancel_error batch down the
+// filter stack.  Yields the call combiner when the batch returns.
+static void yield_call_combiner(grpc_exec_ctx* exec_ctx, void* arg,
+                                grpc_error* ignored) {
+  grpc_deadline_state* deadline_state = arg;
+  GRPC_CALL_COMBINER_STOP(exec_ctx, deadline_state->call_combiner,
+                          "got on_complete from cancel_stream batch");
+  GRPC_CALL_STACK_UNREF(exec_ctx, deadline_state->call_stack, "deadline_timer");
+}
+
+// This is called via the call combiner, so access to deadline_state is
+// synchronized.
+static void send_cancel_op_in_call_combiner(grpc_exec_ctx* exec_ctx, void* arg,
+                                            grpc_error* error) {
+  grpc_call_element* elem = arg;
+  grpc_deadline_state* deadline_state = elem->call_data;
+  grpc_transport_stream_op_batch* batch = grpc_make_transport_stream_op(
+      GRPC_CLOSURE_INIT(&deadline_state->timer_callback, yield_call_combiner,
+                        deadline_state, grpc_schedule_on_exec_ctx));
+  batch->cancel_stream = true;
+  batch->payload->cancel_stream.cancel_error = GRPC_ERROR_REF(error);
+  elem->filter->start_transport_stream_op_batch(exec_ctx, elem, batch);
+}
+
 // Timer callback.
 static void timer_callback(grpc_exec_ctx* exec_ctx, void* arg,
                            grpc_error* error) {
   grpc_call_element* elem = (grpc_call_element*)arg;
   grpc_deadline_state* deadline_state = (grpc_deadline_state*)elem->call_data;
   if (error != GRPC_ERROR_CANCELLED) {
-    grpc_call_element_signal_error(
-        exec_ctx, elem,
-        grpc_error_set_int(
-            GRPC_ERROR_CREATE_FROM_STATIC_STRING("Deadline Exceeded"),
-            GRPC_ERROR_INT_GRPC_STATUS, GRPC_STATUS_DEADLINE_EXCEEDED));
+    error = grpc_error_set_int(
+        GRPC_ERROR_CREATE_FROM_STATIC_STRING("Deadline Exceeded"),
+        GRPC_ERROR_INT_GRPC_STATUS, GRPC_STATUS_DEADLINE_EXCEEDED);
+    grpc_call_combiner_cancel(exec_ctx, deadline_state->call_combiner,
+                              GRPC_ERROR_REF(error));
+    GRPC_CLOSURE_INIT(&deadline_state->timer_callback,
+                      send_cancel_op_in_call_combiner, elem,
+                      grpc_schedule_on_exec_ctx);
+    GRPC_CALL_COMBINER_START(exec_ctx, deadline_state->call_combiner,
+                             &deadline_state->timer_callback, error,
+                             "deadline exceeded -- sending cancel_stream op");
+  } else {
+    GRPC_CALL_STACK_UNREF(exec_ctx, deadline_state->call_stack,
+                          "deadline_timer");
   }
-  GRPC_CALL_STACK_UNREF(exec_ctx, deadline_state->call_stack, "deadline_timer");
 }
 
 // Starts the deadline timer.
+// This is called via the call combiner, so access to deadline_state is
+// synchronized.
 static void start_timer_if_needed(grpc_exec_ctx* exec_ctx,
                                   grpc_call_element* elem,
                                   gpr_timespec deadline) {
@@ -58,51 +92,39 @@ static void start_timer_if_needed(grpc_exec_ctx* exec_ctx,
     return;
   }
   grpc_deadline_state* deadline_state = (grpc_deadline_state*)elem->call_data;
-  grpc_deadline_timer_state cur_state;
   grpc_closure* closure = NULL;
-retry:
-  cur_state =
-      (grpc_deadline_timer_state)gpr_atm_acq_load(&deadline_state->timer_state);
-  switch (cur_state) {
+  switch (deadline_state->timer_state) {
     case GRPC_DEADLINE_STATE_PENDING:
       // Note: We do not start the timer if there is already a timer
       return;
     case GRPC_DEADLINE_STATE_FINISHED:
-      if (gpr_atm_rel_cas(&deadline_state->timer_state,
-                          GRPC_DEADLINE_STATE_FINISHED,
-                          GRPC_DEADLINE_STATE_PENDING)) {
-        // If we've already created and destroyed a timer, we always create a
-        // new closure: we have no other guarantee that the inlined closure is
-        // not in use (it may hold a pending call to timer_callback)
-        closure = GRPC_CLOSURE_CREATE(timer_callback, elem,
-                                      grpc_schedule_on_exec_ctx);
-      } else {
-        goto retry;
-      }
+      deadline_state->timer_state = GRPC_DEADLINE_STATE_PENDING;
+      // If we've already created and destroyed a timer, we always create a
+      // new closure: we have no other guarantee that the inlined closure is
+      // not in use (it may hold a pending call to timer_callback)
+      closure =
+          GRPC_CLOSURE_CREATE(timer_callback, elem, grpc_schedule_on_exec_ctx);
       break;
     case GRPC_DEADLINE_STATE_INITIAL:
-      if (gpr_atm_rel_cas(&deadline_state->timer_state,
-                          GRPC_DEADLINE_STATE_INITIAL,
-                          GRPC_DEADLINE_STATE_PENDING)) {
-        closure =
-            GRPC_CLOSURE_INIT(&deadline_state->timer_callback, timer_callback,
-                              elem, grpc_schedule_on_exec_ctx);
-      } else {
-        goto retry;
-      }
+      deadline_state->timer_state = GRPC_DEADLINE_STATE_PENDING;
+      closure =
+          GRPC_CLOSURE_INIT(&deadline_state->timer_callback, timer_callback,
+                            elem, grpc_schedule_on_exec_ctx);
       break;
   }
-  GPR_ASSERT(closure);
+  GPR_ASSERT(closure != NULL);
   GRPC_CALL_STACK_REF(deadline_state->call_stack, "deadline_timer");
   grpc_timer_init(exec_ctx, &deadline_state->timer, deadline, closure,
                   gpr_now(GPR_CLOCK_MONOTONIC));
 }
 
 // Cancels the deadline timer.
+// This is called via the call combiner, so access to deadline_state is
+// synchronized.
 static void cancel_timer_if_needed(grpc_exec_ctx* exec_ctx,
                                    grpc_deadline_state* deadline_state) {
-  if (gpr_atm_rel_cas(&deadline_state->timer_state, GRPC_DEADLINE_STATE_PENDING,
-                      GRPC_DEADLINE_STATE_FINISHED)) {
+  if (deadline_state->timer_state == GRPC_DEADLINE_STATE_PENDING) {
+    deadline_state->timer_state = GRPC_DEADLINE_STATE_FINISHED;
     grpc_timer_cancel(exec_ctx, &deadline_state->timer);
   } else {
     // timer was either in STATE_INITAL (nothing to cancel)
@@ -131,6 +153,7 @@ static void inject_on_complete_cb(grpc_deadline_state* deadline_state,
 // Callback and associated state for starting the timer after call stack
 // initialization has been completed.
 struct start_timer_after_init_state {
+  bool in_call_combiner;
   grpc_call_element* elem;
   gpr_timespec deadline;
   grpc_closure closure;
@@ -138,15 +161,29 @@ struct start_timer_after_init_state {
 static void start_timer_after_init(grpc_exec_ctx* exec_ctx, void* arg,
                                    grpc_error* error) {
   struct start_timer_after_init_state* state = arg;
+  grpc_deadline_state* deadline_state = state->elem->call_data;
+  if (!state->in_call_combiner) {
+    // We are initially called without holding the call combiner, so we
+    // need to bounce ourselves into it.
+    state->in_call_combiner = true;
+    GRPC_CALL_COMBINER_START(exec_ctx, deadline_state->call_combiner,
+                             &state->closure, GRPC_ERROR_REF(error),
+                             "scheduling deadline timer");
+    return;
+  }
   start_timer_if_needed(exec_ctx, state->elem, state->deadline);
   gpr_free(state);
+  GRPC_CALL_COMBINER_STOP(exec_ctx, deadline_state->call_combiner,
+                          "done scheduling deadline timer");
 }
 
 void grpc_deadline_state_init(grpc_exec_ctx* exec_ctx, grpc_call_element* elem,
                               grpc_call_stack* call_stack,
+                              grpc_call_combiner* call_combiner,
                               gpr_timespec deadline) {
   grpc_deadline_state* deadline_state = (grpc_deadline_state*)elem->call_data;
   deadline_state->call_stack = call_stack;
+  deadline_state->call_combiner = call_combiner;
   // Deadline will always be infinite on servers, so the timer will only be
   // set on clients with a finite deadline.
   deadline = gpr_convert_clock_type(deadline, GPR_CLOCK_MONOTONIC);
@@ -158,7 +195,7 @@ void grpc_deadline_state_init(grpc_exec_ctx* exec_ctx, grpc_call_element* elem,
     // call stack initialization is finished.  To avoid that problem, we
     // create a closure to start the timer, and we schedule that closure
     // to be run after call stack initialization is done.
-    struct start_timer_after_init_state* state = gpr_malloc(sizeof(*state));
+    struct start_timer_after_init_state* state = gpr_zalloc(sizeof(*state));
     state->elem = elem;
     state->deadline = deadline;
     GRPC_CLOSURE_INIT(&state->closure, start_timer_after_init, state,
@@ -232,7 +269,8 @@ typedef struct server_call_data {
 static grpc_error* init_call_elem(grpc_exec_ctx* exec_ctx,
                                   grpc_call_element* elem,
                                   const grpc_call_element_args* args) {
-  grpc_deadline_state_init(exec_ctx, elem, args->call_stack, args->deadline);
+  grpc_deadline_state_init(exec_ctx, elem, args->call_stack,
+                           args->call_combiner, args->deadline);
   return GRPC_ERROR_NONE;
 }
 
@@ -310,7 +348,6 @@ const grpc_channel_filter grpc_client_deadline_filter = {
     0,  // sizeof(channel_data)
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "deadline",
 };
@@ -325,7 +362,6 @@ const grpc_channel_filter grpc_server_deadline_filter = {
     0,  // sizeof(channel_data)
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "deadline",
 };
diff --git a/src/core/ext/filters/deadline/deadline_filter.h b/src/core/ext/filters/deadline/deadline_filter.h
index 420bf7065a..3eb102ad28 100644
--- a/src/core/ext/filters/deadline/deadline_filter.h
+++ b/src/core/ext/filters/deadline/deadline_filter.h
@@ -31,7 +31,8 @@ typedef enum grpc_deadline_timer_state {
 typedef struct grpc_deadline_state {
   // We take a reference to the call stack for the timer callback.
   grpc_call_stack* call_stack;
-  gpr_atm timer_state;
+  grpc_call_combiner* call_combiner;
+  grpc_deadline_timer_state timer_state;
   grpc_timer timer;
   grpc_closure timer_callback;
   // Closure to invoke when the call is complete.
@@ -50,6 +51,7 @@ typedef struct grpc_deadline_state {
 // assumes elem->call_data is zero'd
 void grpc_deadline_state_init(grpc_exec_ctx* exec_ctx, grpc_call_element* elem,
                               grpc_call_stack* call_stack,
+                              grpc_call_combiner* call_combiner,
                               gpr_timespec deadline);
 void grpc_deadline_state_destroy(grpc_exec_ctx* exec_ctx,
                                  grpc_call_element* elem);
@@ -61,6 +63,8 @@ void grpc_deadline_state_destroy(grpc_exec_ctx* exec_ctx,
 // to ensure that the timer callback is not invoked while it is in the
 // process of being reset, which means that attempting to increase the
 // deadline may result in the timer being called twice.
+//
+// Note: Must be called while holding the call combiner.
 void grpc_deadline_state_reset(grpc_exec_ctx* exec_ctx, grpc_call_element* elem,
                                gpr_timespec new_deadline);
 
@@ -70,6 +74,8 @@ void grpc_deadline_state_reset(grpc_exec_ctx* exec_ctx, grpc_call_element* elem,
 //
 // Note: It is the caller's responsibility to chain to the next filter if
 // necessary after this function returns.
+//
+// Note: Must be called while holding the call combiner.
 void grpc_deadline_state_client_start_transport_stream_op_batch(
     grpc_exec_ctx* exec_ctx, grpc_call_element* elem,
     grpc_transport_stream_op_batch* op);
diff --git a/src/core/ext/filters/http/client/http_client_filter.c b/src/core/ext/filters/http/client/http_client_filter.c
index 3ca01a41b5..99ddd08e6a 100644
--- a/src/core/ext/filters/http/client/http_client_filter.c
+++ b/src/core/ext/filters/http/client/http_client_filter.c
@@ -36,6 +36,7 @@
 static const size_t kMaxPayloadSizeForGet = 2048;
 
 typedef struct call_data {
+  grpc_call_combiner *call_combiner;
   // State for handling send_initial_metadata ops.
   grpc_linked_mdelem method;
   grpc_linked_mdelem scheme;
@@ -215,13 +216,13 @@ static void on_send_message_next_done(grpc_exec_ctx *exec_ctx, void *arg,
   call_data *calld = (call_data *)elem->call_data;
   if (error != GRPC_ERROR_NONE) {
     grpc_transport_stream_op_batch_finish_with_failure(
-        exec_ctx, calld->send_message_batch, error);
+        exec_ctx, calld->send_message_batch, error, calld->call_combiner);
     return;
   }
   error = pull_slice_from_send_message(exec_ctx, calld);
   if (error != GRPC_ERROR_NONE) {
     grpc_transport_stream_op_batch_finish_with_failure(
-        exec_ctx, calld->send_message_batch, error);
+        exec_ctx, calld->send_message_batch, error, calld->call_combiner);
     return;
   }
   // There may or may not be more to read, but we don't care.  If we got
@@ -414,7 +415,7 @@ static void hc_start_transport_stream_op_batch(
 done:
   if (error != GRPC_ERROR_NONE) {
     grpc_transport_stream_op_batch_finish_with_failure(
-        exec_ctx, calld->send_message_batch, error);
+        exec_ctx, calld->send_message_batch, error, calld->call_combiner);
   } else if (!batch_will_be_handled_asynchronously) {
     grpc_call_next_op(exec_ctx, elem, batch);
   }
@@ -426,6 +427,7 @@ static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem,
                                   const grpc_call_element_args *args) {
   call_data *calld = (call_data *)elem->call_data;
+  calld->call_combiner = args->call_combiner;
   GRPC_CLOSURE_INIT(&calld->recv_initial_metadata_ready,
                     recv_initial_metadata_ready, elem,
                     grpc_schedule_on_exec_ctx);
@@ -565,6 +567,5 @@ const grpc_channel_filter grpc_http_client_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "http-client"};
diff --git a/src/core/ext/filters/http/message_compress/message_compress_filter.c b/src/core/ext/filters/http/message_compress/message_compress_filter.c
index a32819bfe4..98a503cafc 100644
--- a/src/core/ext/filters/http/message_compress/message_compress_filter.c
+++ b/src/core/ext/filters/http/message_compress/message_compress_filter.c
@@ -35,35 +35,29 @@
 #include "src/core/lib/surface/call.h"
 #include "src/core/lib/transport/static_metadata.h"
 
-#define INITIAL_METADATA_UNSEEN 0
-#define HAS_COMPRESSION_ALGORITHM 2
-#define NO_COMPRESSION_ALGORITHM 4
-
-#define CANCELLED_BIT ((gpr_atm)1)
+typedef enum {
+  // Initial metadata not yet seen.
+  INITIAL_METADATA_UNSEEN = 0,
+  // Initial metadata seen; compression algorithm set.
+  HAS_COMPRESSION_ALGORITHM,
+  // Initial metadata seen; no compression algorithm set.
+  NO_COMPRESSION_ALGORITHM,
+} initial_metadata_state;
 
 typedef struct call_data {
-  grpc_slice_buffer slices; /**< Buffers up input slices to be compressed */
+  grpc_call_combiner *call_combiner;
   grpc_linked_mdelem compression_algorithm_storage;
   grpc_linked_mdelem stream_compression_algorithm_storage;
   grpc_linked_mdelem accept_encoding_storage;
   grpc_linked_mdelem accept_stream_encoding_storage;
-  uint32_t remaining_slice_bytes;
   /** Compression algorithm we'll try to use. It may be given by incoming
    * metadata, or by the channel's default compression settings. */
   grpc_compression_algorithm compression_algorithm;
-
-  /* Atomic recording the state of initial metadata; allowed values:
-     INITIAL_METADATA_UNSEEN - initial metadata op not seen
-     HAS_COMPRESSION_ALGORITHM - initial metadata seen; compression algorithm
-                                 set
-     NO_COMPRESSION_ALGORITHM - initial metadata seen; no compression algorithm
-                                set
-     pointer - a stalled op containing a send_message that's waiting on initial
-               metadata
-     pointer | CANCELLED_BIT - request was cancelled with error pointed to */
-  gpr_atm send_initial_metadata_state;
-
+  initial_metadata_state send_initial_metadata_state;
+  grpc_error *cancel_error;
+  grpc_closure start_send_message_batch_in_call_combiner;
   grpc_transport_stream_op_batch *send_message_batch;
+  grpc_slice_buffer slices; /**< Buffers up input slices to be compressed */
   grpc_slice_buffer_stream replacement_stream;
   grpc_closure *original_send_message_on_complete;
   grpc_closure send_message_on_complete;
@@ -92,13 +86,13 @@ static bool skip_compression(grpc_call_element *elem, uint32_t flags,
   channel_data *channeld = elem->channel_data;
 
   if (flags & (GRPC_WRITE_NO_COMPRESS | GRPC_WRITE_INTERNAL_COMPRESS)) {
-    return 1;
+    return true;
   }
   if (has_compression_algorithm) {
     if (calld->compression_algorithm == GRPC_COMPRESS_NONE) {
-      return 1;
+      return true;
     }
-    return 0; /* we have an actual call-specific algorithm */
+    return false; /* we have an actual call-specific algorithm */
   }
   /* no per-call compression override */
   return channeld->default_compression_algorithm == GRPC_COMPRESS_NONE;
@@ -226,6 +220,18 @@ static void send_message_on_complete(grpc_exec_ctx *exec_ctx, void *arg,
                    GRPC_ERROR_REF(error));
 }
 
+static void send_message_batch_continue(grpc_exec_ctx *exec_ctx,
+                                        grpc_call_element *elem) {
+  call_data *calld = (call_data *)elem->call_data;
+  // Note: The call to grpc_call_next_op() results in yielding the
+  // call combiner, so we need to clear calld->send_message_batch
+  // before we do that.
+  grpc_transport_stream_op_batch *send_message_batch =
+      calld->send_message_batch;
+  calld->send_message_batch = NULL;
+  grpc_call_next_op(exec_ctx, elem, send_message_batch);
+}
+
 static void finish_send_message(grpc_exec_ctx *exec_ctx,
                                 grpc_call_element *elem) {
   call_data *calld = (call_data *)elem->call_data;
@@ -234,8 +240,8 @@ static void finish_send_message(grpc_exec_ctx *exec_ctx,
   grpc_slice_buffer_init(&tmp);
   uint32_t send_flags =
       calld->send_message_batch->payload->send_message.send_message->flags;
-  const bool did_compress = grpc_msg_compress(
-      exec_ctx, calld->compression_algorithm, &calld->slices, &tmp);
+  bool did_compress = grpc_msg_compress(exec_ctx, calld->compression_algorithm,
+                                        &calld->slices, &tmp);
   if (did_compress) {
     if (GRPC_TRACER_ON(grpc_compression_trace)) {
       char *algo_name;
@@ -273,7 +279,19 @@ static void finish_send_message(grpc_exec_ctx *exec_ctx,
   calld->original_send_message_on_complete =
       calld->send_message_batch->on_complete;
   calld->send_message_batch->on_complete = &calld->send_message_on_complete;
-  grpc_call_next_op(exec_ctx, elem, calld->send_message_batch);
+  send_message_batch_continue(exec_ctx, elem);
+}
+
+static void fail_send_message_batch_in_call_combiner(grpc_exec_ctx *exec_ctx,
+                                                     void *arg,
+                                                     grpc_error *error) {
+  call_data *calld = arg;
+  if (calld->send_message_batch != NULL) {
+    grpc_transport_stream_op_batch_finish_with_failure(
+        exec_ctx, calld->send_message_batch, GRPC_ERROR_REF(error),
+        calld->call_combiner);
+    calld->send_message_batch = NULL;
+  }
 }
 
 // Pulls a slice from the send_message byte stream and adds it to calld->slices.
@@ -293,21 +311,25 @@ static grpc_error *pull_slice_from_send_message(grpc_exec_ctx *exec_ctx,
 // If all data has been read, invokes finish_send_message().  Otherwise,
 // an async call to grpc_byte_stream_next() has been started, which will
 // eventually result in calling on_send_message_next_done().
-static grpc_error *continue_reading_send_message(grpc_exec_ctx *exec_ctx,
-                                                 grpc_call_element *elem) {
+static void continue_reading_send_message(grpc_exec_ctx *exec_ctx,
+                                          grpc_call_element *elem) {
   call_data *calld = (call_data *)elem->call_data;
   while (grpc_byte_stream_next(
       exec_ctx, calld->send_message_batch->payload->send_message.send_message,
       ~(size_t)0, &calld->on_send_message_next_done)) {
     grpc_error *error = pull_slice_from_send_message(exec_ctx, calld);
-    if (error != GRPC_ERROR_NONE) return error;
+    if (error != GRPC_ERROR_NONE) {
+      // Closure callback; does not take ownership of error.
+      fail_send_message_batch_in_call_combiner(exec_ctx, calld, error);
+      GRPC_ERROR_UNREF(error);
+      return;
+    }
     if (calld->slices.length ==
         calld->send_message_batch->payload->send_message.send_message->length) {
       finish_send_message(exec_ctx, elem);
       break;
     }
   }
-  return GRPC_ERROR_NONE;
 }
 
 // Async callback for grpc_byte_stream_next().
@@ -315,46 +337,37 @@ static void on_send_message_next_done(grpc_exec_ctx *exec_ctx, void *arg,
                                       grpc_error *error) {
   grpc_call_element *elem = (grpc_call_element *)arg;
   call_data *calld = (call_data *)elem->call_data;
-  if (error != GRPC_ERROR_NONE) goto fail;
+  if (error != GRPC_ERROR_NONE) {
+    // Closure callback; does not take ownership of error.
+    fail_send_message_batch_in_call_combiner(exec_ctx, calld, error);
+    return;
+  }
   error = pull_slice_from_send_message(exec_ctx, calld);
-  if (error != GRPC_ERROR_NONE) goto fail;
+  if (error != GRPC_ERROR_NONE) {
+    // Closure callback; does not take ownership of error.
+    fail_send_message_batch_in_call_combiner(exec_ctx, calld, error);
+    GRPC_ERROR_UNREF(error);
+    return;
+  }
   if (calld->slices.length ==
       calld->send_message_batch->payload->send_message.send_message->length) {
     finish_send_message(exec_ctx, elem);
   } else {
-    // This will either finish reading all of the data and invoke
-    // finish_send_message(), or else it will make an async call to
-    // grpc_byte_stream_next(), which will eventually result in calling
-    // this function again.
-    error = continue_reading_send_message(exec_ctx, elem);
-    if (error != GRPC_ERROR_NONE) goto fail;
+    continue_reading_send_message(exec_ctx, elem);
   }
-  return;
-fail:
-  grpc_transport_stream_op_batch_finish_with_failure(
-      exec_ctx, calld->send_message_batch, error);
 }
 
-static void start_send_message_batch(grpc_exec_ctx *exec_ctx,
-                                     grpc_call_element *elem,
-                                     grpc_transport_stream_op_batch *batch,
-                                     bool has_compression_algorithm) {
+static void start_send_message_batch(grpc_exec_ctx *exec_ctx, void *arg,
+                                     grpc_error *unused) {
+  grpc_call_element *elem = (grpc_call_element *)arg;
   call_data *calld = (call_data *)elem->call_data;
-  if (!skip_compression(elem, batch->payload->send_message.send_message->flags,
-                        has_compression_algorithm)) {
-    calld->send_message_batch = batch;
-    // This will either finish reading all of the data and invoke
-    // finish_send_message(), or else it will make an async call to
-    // grpc_byte_stream_next(), which will eventually result in calling
-    // on_send_message_next_done().
-    grpc_error *error = continue_reading_send_message(exec_ctx, elem);
-    if (error != GRPC_ERROR_NONE) {
-      grpc_transport_stream_op_batch_finish_with_failure(
-          exec_ctx, calld->send_message_batch, error);
-    }
+  if (skip_compression(
+          elem,
+          calld->send_message_batch->payload->send_message.send_message->flags,
+          calld->send_initial_metadata_state == HAS_COMPRESSION_ALGORITHM)) {
+    send_message_batch_continue(exec_ctx, elem);
   } else {
-    /* pass control down the stack */
-    grpc_call_next_op(exec_ctx, elem, batch);
+    continue_reading_send_message(exec_ctx, elem);
   }
 }
 
@@ -362,95 +375,80 @@ static void compress_start_transport_stream_op_batch(
     grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
     grpc_transport_stream_op_batch *batch) {
   call_data *calld = elem->call_data;
-
   GPR_TIMER_BEGIN("compress_start_transport_stream_op_batch", 0);
-
+  // Handle cancel_stream.
   if (batch->cancel_stream) {
-    // TODO(roth): As part of the upcoming call combiner work, change
-    // this to call grpc_byte_stream_shutdown() on the incoming byte
-    // stream, to cancel any in-flight calls to grpc_byte_stream_next().
-    GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error);
-    gpr_atm cur = gpr_atm_full_xchg(
-        &calld->send_initial_metadata_state,
-        CANCELLED_BIT | (gpr_atm)batch->payload->cancel_stream.cancel_error);
-    switch (cur) {
-      case HAS_COMPRESSION_ALGORITHM:
-      case NO_COMPRESSION_ALGORITHM:
-      case INITIAL_METADATA_UNSEEN:
-        break;
-      default:
-        if ((cur & CANCELLED_BIT) == 0) {
-          grpc_transport_stream_op_batch_finish_with_failure(
-              exec_ctx, (grpc_transport_stream_op_batch *)cur,
-              GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error));
-        } else {
-          GRPC_ERROR_UNREF((grpc_error *)(cur & ~CANCELLED_BIT));
-        }
-        break;
+    GRPC_ERROR_UNREF(calld->cancel_error);
+    calld->cancel_error =
+        GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error);
+    if (calld->send_message_batch != NULL) {
+      if (calld->send_initial_metadata_state == INITIAL_METADATA_UNSEEN) {
+        GRPC_CALL_COMBINER_START(
+            exec_ctx, calld->call_combiner,
+            GRPC_CLOSURE_CREATE(fail_send_message_batch_in_call_combiner, calld,
+                                grpc_schedule_on_exec_ctx),
+            GRPC_ERROR_REF(calld->cancel_error), "failing send_message op");
+      } else {
+        grpc_byte_stream_shutdown(
+            exec_ctx,
+            calld->send_message_batch->payload->send_message.send_message,
+            GRPC_ERROR_REF(calld->cancel_error));
+      }
     }
+  } else if (calld->cancel_error != GRPC_ERROR_NONE) {
+    grpc_transport_stream_op_batch_finish_with_failure(
+        exec_ctx, batch, GRPC_ERROR_REF(calld->cancel_error),
+        calld->call_combiner);
+    goto done;
   }
-
+  // Handle send_initial_metadata.
   if (batch->send_initial_metadata) {
+    GPR_ASSERT(calld->send_initial_metadata_state == INITIAL_METADATA_UNSEEN);
     bool has_compression_algorithm;
     grpc_error *error = process_send_initial_metadata(
         exec_ctx, elem,
         batch->payload->send_initial_metadata.send_initial_metadata,
         &has_compression_algorithm);
     if (error != GRPC_ERROR_NONE) {
-      grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch,
-                                                         error);
-      return;
+      grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch, error,
+                                                         calld->call_combiner);
+      goto done;
     }
-    gpr_atm cur;
-  retry_send_im:
-    cur = gpr_atm_acq_load(&calld->send_initial_metadata_state);
-    GPR_ASSERT(cur != HAS_COMPRESSION_ALGORITHM &&
-               cur != NO_COMPRESSION_ALGORITHM);
-    if ((cur & CANCELLED_BIT) == 0) {
-      if (!gpr_atm_rel_cas(&calld->send_initial_metadata_state, cur,
-                           has_compression_algorithm
-                               ? HAS_COMPRESSION_ALGORITHM
-                               : NO_COMPRESSION_ALGORITHM)) {
-        goto retry_send_im;
-      }
-      if (cur != INITIAL_METADATA_UNSEEN) {
-        start_send_message_batch(exec_ctx, elem,
-                                 (grpc_transport_stream_op_batch *)cur,
-                                 has_compression_algorithm);
-      }
+    calld->send_initial_metadata_state = has_compression_algorithm
+                                             ? HAS_COMPRESSION_ALGORITHM
+                                             : NO_COMPRESSION_ALGORITHM;
+    // If we had previously received a batch containing a send_message op,
+    // handle it now.  Note that we need to re-enter the call combiner
+    // for this, since we can't send two batches down while holding the
+    // call combiner, since the connected_channel filter (at the bottom of
+    // the call stack) will release the call combiner for each batch it sees.
+    if (calld->send_message_batch != NULL) {
+      GRPC_CALL_COMBINER_START(
+          exec_ctx, calld->call_combiner,
+          &calld->start_send_message_batch_in_call_combiner, GRPC_ERROR_NONE,
+          "starting send_message after send_initial_metadata");
     }
   }
+  // Handle send_message.
   if (batch->send_message) {
-    gpr_atm cur;
-  retry_send:
-    cur = gpr_atm_acq_load(&calld->send_initial_metadata_state);
-    switch (cur) {
-      case INITIAL_METADATA_UNSEEN:
-        if (!gpr_atm_rel_cas(&calld->send_initial_metadata_state, cur,
-                             (gpr_atm)batch)) {
-          goto retry_send;
-        }
-        break;
-      case HAS_COMPRESSION_ALGORITHM:
-      case NO_COMPRESSION_ALGORITHM:
-        start_send_message_batch(exec_ctx, elem, batch,
-                                 cur == HAS_COMPRESSION_ALGORITHM);
-        break;
-      default:
-        if (cur & CANCELLED_BIT) {
-          grpc_transport_stream_op_batch_finish_with_failure(
-              exec_ctx, batch,
-              GRPC_ERROR_REF((grpc_error *)(cur & ~CANCELLED_BIT)));
-        } else {
-          /* >1 send_message concurrently */
-          GPR_UNREACHABLE_CODE(break);
-        }
+    GPR_ASSERT(calld->send_message_batch == NULL);
+    calld->send_message_batch = batch;
+    // If we have not yet seen send_initial_metadata, then we have to
+    // wait.  We save the batch in calld and then drop the call
+    // combiner, which we'll have to pick up again later when we get
+    // send_initial_metadata.
+    if (calld->send_initial_metadata_state == INITIAL_METADATA_UNSEEN) {
+      GRPC_CALL_COMBINER_STOP(
+          exec_ctx, calld->call_combiner,
+          "send_message batch pending send_initial_metadata");
+      goto done;
     }
+    start_send_message_batch(exec_ctx, elem, GRPC_ERROR_NONE);
   } else {
-    /* pass control down the stack */
+    // Pass control down the stack.
     grpc_call_next_op(exec_ctx, elem, batch);
   }
-
+done:
   GPR_TIMER_END("compress_start_transport_stream_op_batch", 0);
 }
 
@@ -458,16 +456,16 @@ static void compress_start_transport_stream_op_batch(
 static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem,
                                   const grpc_call_element_args *args) {
-  /* grab pointers to our data from the call element */
-  call_data *calld = elem->call_data;
-
-  /* initialize members */
+  call_data *calld = (call_data *)elem->call_data;
+  calld->call_combiner = args->call_combiner;
+  calld->cancel_error = GRPC_ERROR_NONE;
   grpc_slice_buffer_init(&calld->slices);
+  GRPC_CLOSURE_INIT(&calld->start_send_message_batch_in_call_combiner,
+                    start_send_message_batch, elem, grpc_schedule_on_exec_ctx);
   GRPC_CLOSURE_INIT(&calld->on_send_message_next_done,
                     on_send_message_next_done, elem, grpc_schedule_on_exec_ctx);
   GRPC_CLOSURE_INIT(&calld->send_message_on_complete, send_message_on_complete,
                     elem, grpc_schedule_on_exec_ctx);
-
   return GRPC_ERROR_NONE;
 }
 
@@ -475,14 +473,9 @@ static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
 static void destroy_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
                               const grpc_call_final_info *final_info,
                               grpc_closure *ignored) {
-  /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
   grpc_slice_buffer_destroy_internal(exec_ctx, &calld->slices);
-  gpr_atm imstate =
-      gpr_atm_no_barrier_load(&calld->send_initial_metadata_state);
-  if (imstate & CANCELLED_BIT) {
-    GRPC_ERROR_UNREF((grpc_error *)(imstate & ~CANCELLED_BIT));
-  }
+  GRPC_ERROR_UNREF(calld->cancel_error);
 }
 
 /* Constructor for channel_data */
@@ -550,6 +543,5 @@ const grpc_channel_filter grpc_message_compress_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
-    "compress"};
+    "message_compress"};
diff --git a/src/core/ext/filters/http/server/http_server_filter.c b/src/core/ext/filters/http/server/http_server_filter.c
index b145f12aff..a10e69ba59 100644
--- a/src/core/ext/filters/http/server/http_server_filter.c
+++ b/src/core/ext/filters/http/server/http_server_filter.c
@@ -32,6 +32,8 @@
 #define EXPECTED_CONTENT_TYPE_LENGTH sizeof(EXPECTED_CONTENT_TYPE) - 1
 
 typedef struct call_data {
+  grpc_call_combiner *call_combiner;
+
   grpc_linked_mdelem status;
   grpc_linked_mdelem content_type;
 
@@ -281,7 +283,11 @@ static void hs_on_complete(grpc_exec_ctx *exec_ctx, void *user_data,
     *calld->pp_recv_message = calld->payload_bin_delivered
                                   ? NULL
                                   : (grpc_byte_stream *)&calld->read_stream;
-    GRPC_CLOSURE_RUN(exec_ctx, calld->recv_message_ready, GRPC_ERROR_REF(err));
+    // Re-enter call combiner for recv_message_ready, since the surface
+    // code will release the call combiner for each callback it receives.
+    GRPC_CALL_COMBINER_START(exec_ctx, calld->call_combiner,
+                             calld->recv_message_ready, GRPC_ERROR_REF(err),
+                             "resuming recv_message_ready from on_complete");
     calld->recv_message_ready = NULL;
     calld->payload_bin_delivered = true;
   }
@@ -293,15 +299,20 @@ static void hs_recv_message_ready(grpc_exec_ctx *exec_ctx, void *user_data,
   grpc_call_element *elem = user_data;
   call_data *calld = elem->call_data;
   if (calld->seen_path_with_query) {
-    /* do nothing. This is probably a GET request, and payload will be returned
-    in hs_on_complete callback. */
+    // Do nothing. This is probably a GET request, and payload will be
+    // returned in hs_on_complete callback.
+    // Note that we release the call combiner here, so that other
+    // callbacks can run.
+    GRPC_CALL_COMBINER_STOP(exec_ctx, calld->call_combiner,
+                            "pausing recv_message_ready until on_complete");
   } else {
     GRPC_CLOSURE_RUN(exec_ctx, calld->recv_message_ready, GRPC_ERROR_REF(err));
   }
 }
 
-static void hs_mutate_op(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                         grpc_transport_stream_op_batch *op) {
+static grpc_error *hs_mutate_op(grpc_exec_ctx *exec_ctx,
+                                grpc_call_element *elem,
+                                grpc_transport_stream_op_batch *op) {
   /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
 
@@ -323,10 +334,7 @@ static void hs_mutate_op(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
               server_filter_outgoing_metadata(
                   exec_ctx, elem,
                   op->payload->send_initial_metadata.send_initial_metadata));
-    if (error != GRPC_ERROR_NONE) {
-      grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, op, error);
-      return;
-    }
+    if (error != GRPC_ERROR_NONE) return error;
   }
 
   if (op->recv_initial_metadata) {
@@ -359,21 +367,25 @@ static void hs_mutate_op(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
     grpc_error *error = server_filter_outgoing_metadata(
         exec_ctx, elem,
         op->payload->send_trailing_metadata.send_trailing_metadata);
-    if (error != GRPC_ERROR_NONE) {
-      grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, op, error);
-      return;
-    }
+    if (error != GRPC_ERROR_NONE) return error;
   }
+
+  return GRPC_ERROR_NONE;
 }
 
-static void hs_start_transport_op(grpc_exec_ctx *exec_ctx,
-                                  grpc_call_element *elem,
-                                  grpc_transport_stream_op_batch *op) {
-  GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
-  GPR_TIMER_BEGIN("hs_start_transport_op", 0);
-  hs_mutate_op(exec_ctx, elem, op);
-  grpc_call_next_op(exec_ctx, elem, op);
-  GPR_TIMER_END("hs_start_transport_op", 0);
+static void hs_start_transport_stream_op_batch(
+    grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
+    grpc_transport_stream_op_batch *op) {
+  call_data *calld = elem->call_data;
+  GPR_TIMER_BEGIN("hs_start_transport_stream_op_batch", 0);
+  grpc_error *error = hs_mutate_op(exec_ctx, elem, op);
+  if (error != GRPC_ERROR_NONE) {
+    grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, op, error,
+                                                       calld->call_combiner);
+  } else {
+    grpc_call_next_op(exec_ctx, elem, op);
+  }
+  GPR_TIMER_END("hs_start_transport_stream_op_batch", 0);
 }
 
 /* Constructor for call_data */
@@ -383,6 +395,7 @@ static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
   /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
   /* initialize members */
+  calld->call_combiner = args->call_combiner;
   GRPC_CLOSURE_INIT(&calld->hs_on_recv, hs_on_recv, elem,
                     grpc_schedule_on_exec_ctx);
   GRPC_CLOSURE_INIT(&calld->hs_on_complete, hs_on_complete, elem,
@@ -414,7 +427,7 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
                                  grpc_channel_element *elem) {}
 
 const grpc_channel_filter grpc_http_server_filter = {
-    hs_start_transport_op,
+    hs_start_transport_stream_op_batch,
     grpc_channel_next_op,
     sizeof(call_data),
     init_call_elem,
@@ -423,6 +436,5 @@ const grpc_channel_filter grpc_http_server_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "http-server"};
diff --git a/src/core/ext/filters/load_reporting/load_reporting_filter.c b/src/core/ext/filters/load_reporting/load_reporting_filter.c
index 08474efb2e..17e946937f 100644
--- a/src/core/ext/filters/load_reporting/load_reporting_filter.c
+++ b/src/core/ext/filters/load_reporting/load_reporting_filter.c
@@ -223,6 +223,5 @@ const grpc_channel_filter grpc_load_reporting_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "load_reporting"};
diff --git a/src/core/ext/filters/max_age/max_age_filter.c b/src/core/ext/filters/max_age/max_age_filter.c
index 7d748b9c32..16c85a70d0 100644
--- a/src/core/ext/filters/max_age/max_age_filter.c
+++ b/src/core/ext/filters/max_age/max_age_filter.c
@@ -391,7 +391,6 @@ const grpc_channel_filter grpc_max_age_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "max_age"};
 
diff --git a/src/core/ext/filters/message_size/message_size_filter.c b/src/core/ext/filters/message_size/message_size_filter.c
index 846c7df69a..47763b1deb 100644
--- a/src/core/ext/filters/message_size/message_size_filter.c
+++ b/src/core/ext/filters/message_size/message_size_filter.c
@@ -68,6 +68,7 @@ static void* message_size_limits_create_from_json(const grpc_json* json) {
 }
 
 typedef struct call_data {
+  grpc_call_combiner* call_combiner;
   message_size_limits limits;
   // Receive closures are chained: we inject this closure as the
   // recv_message_ready up-call on transport_stream_op, and remember to
@@ -131,7 +132,8 @@ static void start_transport_stream_op_batch(
         exec_ctx, op,
         grpc_error_set_int(GRPC_ERROR_CREATE_FROM_COPIED_STRING(message_string),
                            GRPC_ERROR_INT_GRPC_STATUS,
-                           GRPC_STATUS_RESOURCE_EXHAUSTED));
+                           GRPC_STATUS_RESOURCE_EXHAUSTED),
+        calld->call_combiner);
     gpr_free(message_string);
     return;
   }
@@ -152,6 +154,7 @@ static grpc_error* init_call_elem(grpc_exec_ctx* exec_ctx,
                                   const grpc_call_element_args* args) {
   channel_data* chand = (channel_data*)elem->channel_data;
   call_data* calld = (call_data*)elem->call_data;
+  calld->call_combiner = args->call_combiner;
   calld->next_recv_message_ready = NULL;
   GRPC_CLOSURE_INIT(&calld->recv_message_ready, recv_message_ready, elem,
                     grpc_schedule_on_exec_ctx);
@@ -259,7 +262,6 @@ const grpc_channel_filter grpc_message_size_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "message_size"};
 
diff --git a/src/core/ext/filters/workarounds/workaround_cronet_compression_filter.c b/src/core/ext/filters/workarounds/workaround_cronet_compression_filter.c
index b4d2cb4b8c..c8b2fe5f99 100644
--- a/src/core/ext/filters/workarounds/workaround_cronet_compression_filter.c
+++ b/src/core/ext/filters/workarounds/workaround_cronet_compression_filter.c
@@ -177,7 +177,6 @@ const grpc_channel_filter grpc_workaround_cronet_compression_filter = {
     0,
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "workaround_cronet_compression"};
 
diff --git a/src/core/ext/transport/chttp2/transport/chttp2_transport.c b/src/core/ext/transport/chttp2/transport/chttp2_transport.c
index 7541bd5c92..2f0ac85152 100644
--- a/src/core/ext/transport/chttp2/transport/chttp2_transport.c
+++ b/src/core/ext/transport/chttp2/transport/chttp2_transport.c
@@ -1370,17 +1370,28 @@ static void perform_stream_op_locked(grpc_exec_ctx *exec_ctx, void *stream_op,
             "send_initial_metadata_finished");
       }
     }
+    if (op_payload->send_initial_metadata.peer_string != NULL) {
+      gpr_atm_rel_store(op_payload->send_initial_metadata.peer_string,
+                        (gpr_atm)gpr_strdup(t->peer_string));
+    }
   }
 
   if (op->send_message) {
     on_complete->next_data.scratch |= CLOSURE_BARRIER_MAY_COVER_WRITE;
     s->fetching_send_message_finished = add_closure_barrier(op->on_complete);
     if (s->write_closed) {
+      // Return an error unless the client has already received trailing
+      // metadata from the server, since an application using a
+      // streaming call might send another message before getting a
+      // recv_message failure, breaking out of its loop, and then
+      // starting recv_trailing_metadata.
       grpc_chttp2_complete_closure_step(
           exec_ctx, t, s, &s->fetching_send_message_finished,
-          GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
-              "Attempt to send message after stream was closed",
-              &s->write_closed_error, 1),
+          t->is_client && s->received_trailing_metadata
+              ? GRPC_ERROR_NONE
+              : GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
+                    "Attempt to send message after stream was closed",
+                    &s->write_closed_error, 1),
           "fetching_send_message_finished");
     } else {
       GPR_ASSERT(s->fetching_send_message == NULL);
@@ -1466,6 +1477,10 @@ static void perform_stream_op_locked(grpc_exec_ctx *exec_ctx, void *stream_op,
         op_payload->recv_initial_metadata.recv_initial_metadata;
     s->trailing_metadata_available =
         op_payload->recv_initial_metadata.trailing_metadata_available;
+    if (op_payload->recv_initial_metadata.peer_string != NULL) {
+      gpr_atm_rel_store(op_payload->recv_initial_metadata.peer_string,
+                        (gpr_atm)gpr_strdup(t->peer_string));
+    }
     grpc_chttp2_maybe_complete_recv_initial_metadata(exec_ctx, t, s);
   }
 
@@ -1824,8 +1839,7 @@ void grpc_chttp2_maybe_complete_recv_trailing_metadata(grpc_exec_ctx *exec_ctx,
         }
       }
     }
-    if (s->read_closed && s->frame_storage.length == 0 &&
-        (!pending_data || s->seen_error) &&
+    if (s->read_closed && s->frame_storage.length == 0 && !pending_data &&
         s->recv_trailing_metadata_finished != NULL) {
       grpc_chttp2_incoming_metadata_buffer_publish(
           exec_ctx, &s->metadata_buffer[1], s->recv_trailing_metadata);
@@ -2932,14 +2946,6 @@ static void destructive_reclaimer_locked(grpc_exec_ctx *exec_ctx, void *arg,
 }
 
 /*******************************************************************************
- * INTEGRATION GLUE
- */
-
-static char *chttp2_get_peer(grpc_exec_ctx *exec_ctx, grpc_transport *t) {
-  return gpr_strdup(((grpc_chttp2_transport *)t)->peer_string);
-}
-
-/*******************************************************************************
  * MONITORING
  */
 static grpc_endpoint *chttp2_get_endpoint(grpc_exec_ctx *exec_ctx,
@@ -2956,7 +2962,6 @@ static const grpc_transport_vtable vtable = {sizeof(grpc_chttp2_stream),
                                              perform_transport_op,
                                              destroy_stream,
                                              destroy_transport,
-                                             chttp2_get_peer,
                                              chttp2_get_endpoint};
 
 grpc_transport *grpc_create_chttp2_transport(
diff --git a/src/core/ext/transport/chttp2/transport/internal.h b/src/core/ext/transport/chttp2/transport/internal.h
index 3c41a8958f..9fff30d54f 100644
--- a/src/core/ext/transport/chttp2/transport/internal.h
+++ b/src/core/ext/transport/chttp2/transport/internal.h
@@ -509,6 +509,8 @@ struct grpc_chttp2_stream {
   /** Are we buffering writes on this stream? If yes, we won't become writable
       until there's enough queued up in the flow_controlled_buffer */
   bool write_buffering;
+  /** Has trailing metadata been received. */
+  bool received_trailing_metadata;
 
   /** the error that resulted in this stream being read-closed */
   grpc_error *read_closed_error;
diff --git a/src/core/ext/transport/chttp2/transport/parsing.c b/src/core/ext/transport/chttp2/transport/parsing.c
index 18d163ee98..19bd86fd0c 100644
--- a/src/core/ext/transport/chttp2/transport/parsing.c
+++ b/src/core/ext/transport/chttp2/transport/parsing.c
@@ -623,6 +623,7 @@ static grpc_error *init_header_frame_parser(grpc_exec_ctx *exec_ctx,
           *s->trailing_metadata_available = true;
         }
         t->hpack_parser.on_header = on_trailing_header;
+        s->received_trailing_metadata = true;
       } else {
         GRPC_CHTTP2_IF_TRACING(gpr_log(GPR_INFO, "parsing initial_metadata"));
         t->hpack_parser.on_header = on_initial_header;
@@ -631,6 +632,7 @@ static grpc_error *init_header_frame_parser(grpc_exec_ctx *exec_ctx,
     case 1:
       GRPC_CHTTP2_IF_TRACING(gpr_log(GPR_INFO, "parsing trailing_metadata"));
       t->hpack_parser.on_header = on_trailing_header;
+      s->received_trailing_metadata = true;
       break;
     case 2:
       gpr_log(GPR_ERROR, "too many header frames received");
diff --git a/src/core/ext/transport/cronet/transport/cronet_transport.c b/src/core/ext/transport/cronet/transport/cronet_transport.c
index abb558982b..09420d92e7 100644
--- a/src/core/ext/transport/cronet/transport/cronet_transport.c
+++ b/src/core/ext/transport/cronet/transport/cronet_transport.c
@@ -1386,10 +1386,6 @@ static void destroy_stream(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
 
 static void destroy_transport(grpc_exec_ctx *exec_ctx, grpc_transport *gt) {}
 
-static char *get_peer(grpc_exec_ctx *exec_ctx, grpc_transport *gt) {
-  return NULL;
-}
-
 static grpc_endpoint *get_endpoint(grpc_exec_ctx *exec_ctx,
                                    grpc_transport *gt) {
   return NULL;
@@ -1408,7 +1404,6 @@ static const grpc_transport_vtable grpc_cronet_vtable = {
     perform_op,
     destroy_stream,
     destroy_transport,
-    get_peer,
     get_endpoint};
 
 grpc_transport *grpc_create_cronet_transport(void *engine, const char *target,
diff --git a/src/core/ext/transport/inproc/inproc_transport.c b/src/core/ext/transport/inproc/inproc_transport.c
index 6f4b429ee2..b2d6f2d0c9 100644
--- a/src/core/ext/transport/inproc/inproc_transport.c
+++ b/src/core/ext/transport/inproc/inproc_transport.c
@@ -1251,20 +1251,14 @@ static void set_pollset_set(grpc_exec_ctx *exec_ctx, grpc_transport *gt,
   // Nothing to do here
 }
 
-static char *get_peer(grpc_exec_ctx *exec_ctx, grpc_transport *t) {
-  return gpr_strdup("inproc");
-}
-
 static grpc_endpoint *get_endpoint(grpc_exec_ctx *exec_ctx, grpc_transport *t) {
   return NULL;
 }
 
 static const grpc_transport_vtable inproc_vtable = {
-    sizeof(inproc_stream), "inproc",
-    init_stream,           set_pollset,
-    set_pollset_set,       perform_stream_op,
-    perform_transport_op,  destroy_stream,
-    destroy_transport,     get_peer,
+    sizeof(inproc_stream), "inproc",        init_stream,
+    set_pollset,           set_pollset_set, perform_stream_op,
+    perform_transport_op,  destroy_stream,  destroy_transport,
     get_endpoint};
 
 /*******************************************************************************
diff --git a/src/core/lib/channel/channel_stack.c b/src/core/lib/channel/channel_stack.c
index 0f8e33c4be..775c8bc667 100644
--- a/src/core/lib/channel/channel_stack.c
+++ b/src/core/lib/channel/channel_stack.c
@@ -233,15 +233,10 @@ void grpc_call_stack_destroy(grpc_exec_ctx *exec_ctx, grpc_call_stack *stack,
 void grpc_call_next_op(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
                        grpc_transport_stream_op_batch *op) {
   grpc_call_element *next_elem = elem + 1;
+  GRPC_CALL_LOG_OP(GPR_INFO, next_elem, op);
   next_elem->filter->start_transport_stream_op_batch(exec_ctx, next_elem, op);
 }
 
-char *grpc_call_next_get_peer(grpc_exec_ctx *exec_ctx,
-                              grpc_call_element *elem) {
-  grpc_call_element *next_elem = elem + 1;
-  return next_elem->filter->get_peer(exec_ctx, next_elem);
-}
-
 void grpc_channel_next_get_info(grpc_exec_ctx *exec_ctx,
                                 grpc_channel_element *elem,
                                 const grpc_channel_info *channel_info) {
@@ -265,12 +260,3 @@ grpc_call_stack *grpc_call_stack_from_top_element(grpc_call_element *elem) {
   return (grpc_call_stack *)((char *)(elem)-ROUND_UP_TO_ALIGNMENT_SIZE(
       sizeof(grpc_call_stack)));
 }
-
-void grpc_call_element_signal_error(grpc_exec_ctx *exec_ctx,
-                                    grpc_call_element *elem,
-                                    grpc_error *error) {
-  grpc_transport_stream_op_batch *op = grpc_make_transport_stream_op(NULL);
-  op->cancel_stream = true;
-  op->payload->cancel_stream.cancel_error = error;
-  elem->filter->start_transport_stream_op_batch(exec_ctx, elem, op);
-}
diff --git a/src/core/lib/channel/channel_stack.h b/src/core/lib/channel/channel_stack.h
index a80f8aa826..ae1cac31f7 100644
--- a/src/core/lib/channel/channel_stack.h
+++ b/src/core/lib/channel/channel_stack.h
@@ -40,6 +40,7 @@
 #include <grpc/support/time.h>
 
 #include "src/core/lib/debug/trace.h"
+#include "src/core/lib/iomgr/call_combiner.h"
 #include "src/core/lib/iomgr/polling_entity.h"
 #include "src/core/lib/support/arena.h"
 #include "src/core/lib/transport/transport.h"
@@ -71,6 +72,7 @@ typedef struct {
   gpr_timespec start_time;
   gpr_timespec deadline;
   gpr_arena *arena;
+  grpc_call_combiner *call_combiner;
 } grpc_call_element_args;
 
 typedef struct {
@@ -150,9 +152,6 @@ typedef struct {
   void (*destroy_channel_elem)(grpc_exec_ctx *exec_ctx,
                                grpc_channel_element *elem);
 
-  /* Implement grpc_call_get_peer() */
-  char *(*get_peer)(grpc_exec_ctx *exec_ctx, grpc_call_element *elem);
-
   /* Implement grpc_channel_get_info() */
   void (*get_channel_info)(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
                            const grpc_channel_info *channel_info);
@@ -271,8 +270,6 @@ void grpc_call_next_op(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
    stack */
 void grpc_channel_next_op(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
                           grpc_transport_op *op);
-/* Pass through a request to get_peer to the next child element */
-char *grpc_call_next_get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem);
 /* Pass through a request to get_channel_info() to the next child element */
 void grpc_channel_next_get_info(grpc_exec_ctx *exec_ctx,
                                 grpc_channel_element *elem,
@@ -288,10 +285,6 @@ void grpc_call_log_op(char *file, int line, gpr_log_severity severity,
                       grpc_call_element *elem,
                       grpc_transport_stream_op_batch *op);
 
-void grpc_call_element_signal_error(grpc_exec_ctx *exec_ctx,
-                                    grpc_call_element *cur_elem,
-                                    grpc_error *error);
-
 extern grpc_tracer_flag grpc_trace_channel;
 
 #define GRPC_CALL_LOG_OP(sev, elem, op) \
diff --git a/src/core/lib/channel/connected_channel.c b/src/core/lib/channel/connected_channel.c
index af06ca802e..8285226fc4 100644
--- a/src/core/lib/channel/connected_channel.c
+++ b/src/core/lib/channel/connected_channel.c
@@ -36,7 +36,57 @@ typedef struct connected_channel_channel_data {
   grpc_transport *transport;
 } channel_data;
 
-typedef struct connected_channel_call_data { void *unused; } call_data;
+typedef struct {
+  grpc_closure closure;
+  grpc_closure *original_closure;
+  grpc_call_combiner *call_combiner;
+  const char *reason;
+} callback_state;
+
+typedef struct connected_channel_call_data {
+  grpc_call_combiner *call_combiner;
+  // Closures used for returning results on the call combiner.
+  callback_state on_complete[6];  // Max number of pending batches.
+  callback_state recv_initial_metadata_ready;
+  callback_state recv_message_ready;
+} call_data;
+
+static void run_in_call_combiner(grpc_exec_ctx *exec_ctx, void *arg,
+                                 grpc_error *error) {
+  callback_state *state = (callback_state *)arg;
+  GRPC_CALL_COMBINER_START(exec_ctx, state->call_combiner,
+                           state->original_closure, GRPC_ERROR_REF(error),
+                           state->reason);
+}
+
+static void run_cancel_in_call_combiner(grpc_exec_ctx *exec_ctx, void *arg,
+                                        grpc_error *error) {
+  run_in_call_combiner(exec_ctx, arg, error);
+  gpr_free(arg);
+}
+
+static void intercept_callback(call_data *calld, callback_state *state,
+                               bool free_when_done, const char *reason,
+                               grpc_closure **original_closure) {
+  state->original_closure = *original_closure;
+  state->call_combiner = calld->call_combiner;
+  state->reason = reason;
+  *original_closure = GRPC_CLOSURE_INIT(
+      &state->closure,
+      free_when_done ? run_cancel_in_call_combiner : run_in_call_combiner,
+      state, grpc_schedule_on_exec_ctx);
+}
+
+static callback_state *get_state_for_batch(
+    call_data *calld, grpc_transport_stream_op_batch *batch) {
+  if (batch->send_initial_metadata) return &calld->on_complete[0];
+  if (batch->send_message) return &calld->on_complete[1];
+  if (batch->send_trailing_metadata) return &calld->on_complete[2];
+  if (batch->recv_initial_metadata) return &calld->on_complete[3];
+  if (batch->recv_message) return &calld->on_complete[4];
+  if (batch->recv_trailing_metadata) return &calld->on_complete[5];
+  GPR_UNREACHABLE_CODE(return NULL);
+}
 
 /* We perform a small hack to locate transport data alongside the connected
    channel data in call allocations, to allow everything to be pulled in minimal
@@ -49,13 +99,38 @@ typedef struct connected_channel_call_data { void *unused; } call_data;
    into transport stream operations */
 static void con_start_transport_stream_op_batch(
     grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-    grpc_transport_stream_op_batch *op) {
+    grpc_transport_stream_op_batch *batch) {
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
-  GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
-
+  if (batch->recv_initial_metadata) {
+    callback_state *state = &calld->recv_initial_metadata_ready;
+    intercept_callback(
+        calld, state, false, "recv_initial_metadata_ready",
+        &batch->payload->recv_initial_metadata.recv_initial_metadata_ready);
+  }
+  if (batch->recv_message) {
+    callback_state *state = &calld->recv_message_ready;
+    intercept_callback(calld, state, false, "recv_message_ready",
+                       &batch->payload->recv_message.recv_message_ready);
+  }
+  if (batch->cancel_stream) {
+    // There can be more than one cancellation batch in flight at any
+    // given time, so we can't just pick out a fixed index into
+    // calld->on_complete like we can for the other ops.  However,
+    // cancellation isn't in the fast path, so we just allocate a new
+    // closure for each one.
+    callback_state *state = (callback_state *)gpr_malloc(sizeof(*state));
+    intercept_callback(calld, state, true, "on_complete (cancel_stream)",
+                       &batch->on_complete);
+  } else {
+    callback_state *state = get_state_for_batch(calld, batch);
+    intercept_callback(calld, state, false, "on_complete", &batch->on_complete);
+  }
   grpc_transport_perform_stream_op(exec_ctx, chand->transport,
-                                   TRANSPORT_STREAM_FROM_CALL_DATA(calld), op);
+                                   TRANSPORT_STREAM_FROM_CALL_DATA(calld),
+                                   batch);
+  GRPC_CALL_COMBINER_STOP(exec_ctx, calld->call_combiner,
+                          "passed batch to transport");
 }
 
 static void con_start_transport_op(grpc_exec_ctx *exec_ctx,
@@ -71,6 +146,7 @@ static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
                                   const grpc_call_element_args *args) {
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
+  calld->call_combiner = args->call_combiner;
   int r = grpc_transport_init_stream(
       exec_ctx, chand->transport, TRANSPORT_STREAM_FROM_CALL_DATA(calld),
       &args->call_stack->refcount, args->server_transport_data, args->arena);
@@ -118,11 +194,6 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
   }
 }
 
-static char *con_get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  channel_data *chand = elem->channel_data;
-  return grpc_transport_get_peer(exec_ctx, chand->transport);
-}
-
 /* No-op. */
 static void con_get_channel_info(grpc_exec_ctx *exec_ctx,
                                  grpc_channel_element *elem,
@@ -138,7 +209,6 @@ const grpc_channel_filter grpc_connected_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    con_get_peer,
     con_get_channel_info,
     "connected",
 };
diff --git a/src/core/lib/iomgr/call_combiner.c b/src/core/lib/iomgr/call_combiner.c
new file mode 100644
index 0000000000..899f98552d
--- /dev/null
+++ b/src/core/lib/iomgr/call_combiner.c
@@ -0,0 +1,180 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "src/core/lib/iomgr/call_combiner.h"
+
+#include <grpc/support/log.h>
+
+grpc_tracer_flag grpc_call_combiner_trace =
+    GRPC_TRACER_INITIALIZER(false, "call_combiner");
+
+static grpc_error* decode_cancel_state_error(gpr_atm cancel_state) {
+  if (cancel_state & 1) {
+    return (grpc_error*)(cancel_state & ~(gpr_atm)1);
+  }
+  return GRPC_ERROR_NONE;
+}
+
+static gpr_atm encode_cancel_state_error(grpc_error* error) {
+  return (gpr_atm)1 | (gpr_atm)error;
+}
+
+void grpc_call_combiner_init(grpc_call_combiner* call_combiner) {
+  gpr_mpscq_init(&call_combiner->queue);
+}
+
+void grpc_call_combiner_destroy(grpc_call_combiner* call_combiner) {
+  gpr_mpscq_destroy(&call_combiner->queue);
+  GRPC_ERROR_UNREF(decode_cancel_state_error(call_combiner->cancel_state));
+}
+
+#ifndef NDEBUG
+#define DEBUG_ARGS , const char *file, int line
+#define DEBUG_FMT_STR "%s:%d: "
+#define DEBUG_FMT_ARGS , file, line
+#else
+#define DEBUG_ARGS
+#define DEBUG_FMT_STR
+#define DEBUG_FMT_ARGS
+#endif
+
+void grpc_call_combiner_start(grpc_exec_ctx* exec_ctx,
+                              grpc_call_combiner* call_combiner,
+                              grpc_closure* closure,
+                              grpc_error* error DEBUG_ARGS,
+                              const char* reason) {
+  if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+    gpr_log(GPR_DEBUG,
+            "==> grpc_call_combiner_start() [%p] closure=%p [" DEBUG_FMT_STR
+            "%s] error=%s",
+            call_combiner, closure DEBUG_FMT_ARGS, reason,
+            grpc_error_string(error));
+  }
+  size_t prev_size =
+      (size_t)gpr_atm_full_fetch_add(&call_combiner->size, (gpr_atm)1);
+  if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+    gpr_log(GPR_DEBUG, "  size: %" PRIdPTR " -> %" PRIdPTR, prev_size,
+            prev_size + 1);
+  }
+  if (prev_size == 0) {
+    if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+      gpr_log(GPR_DEBUG, "  EXECUTING IMMEDIATELY");
+    }
+    // Queue was empty, so execute this closure immediately.
+    GRPC_CLOSURE_SCHED(exec_ctx, closure, error);
+  } else {
+    if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+      gpr_log(GPR_INFO, "  QUEUING");
+    }
+    // Queue was not empty, so add closure to queue.
+    closure->error_data.error = error;
+    gpr_mpscq_push(&call_combiner->queue, (gpr_mpscq_node*)closure);
+  }
+}
+
+void grpc_call_combiner_stop(grpc_exec_ctx* exec_ctx,
+                             grpc_call_combiner* call_combiner DEBUG_ARGS,
+                             const char* reason) {
+  if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+    gpr_log(GPR_DEBUG,
+            "==> grpc_call_combiner_stop() [%p] [" DEBUG_FMT_STR "%s]",
+            call_combiner DEBUG_FMT_ARGS, reason);
+  }
+  size_t prev_size =
+      (size_t)gpr_atm_full_fetch_add(&call_combiner->size, (gpr_atm)-1);
+  if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+    gpr_log(GPR_DEBUG, "  size: %" PRIdPTR " -> %" PRIdPTR, prev_size,
+            prev_size - 1);
+  }
+  GPR_ASSERT(prev_size >= 1);
+  if (prev_size > 1) {
+    while (true) {
+      if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+        gpr_log(GPR_DEBUG, "  checking queue");
+      }
+      bool empty;
+      grpc_closure* closure = (grpc_closure*)gpr_mpscq_pop_and_check_end(
+          &call_combiner->queue, &empty);
+      if (closure == NULL) {
+        // This can happen either due to a race condition within the mpscq
+        // code or because of a race with grpc_call_combiner_start().
+        if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+          gpr_log(GPR_DEBUG, "  queue returned no result; checking again");
+        }
+        continue;
+      }
+      if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+        gpr_log(GPR_DEBUG, "  EXECUTING FROM QUEUE: closure=%p error=%s",
+                closure, grpc_error_string(closure->error_data.error));
+      }
+      GRPC_CLOSURE_SCHED(exec_ctx, closure, closure->error_data.error);
+      break;
+    }
+  } else if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+    gpr_log(GPR_DEBUG, "  queue empty");
+  }
+}
+
+void grpc_call_combiner_set_notify_on_cancel(grpc_exec_ctx* exec_ctx,
+                                             grpc_call_combiner* call_combiner,
+                                             grpc_closure* closure) {
+  while (true) {
+    // Decode original state.
+    gpr_atm original_state = gpr_atm_acq_load(&call_combiner->cancel_state);
+    grpc_error* original_error = decode_cancel_state_error(original_state);
+    // If error is set, invoke the cancellation closure immediately.
+    // Otherwise, store the new closure.
+    if (original_error != GRPC_ERROR_NONE) {
+      GRPC_CLOSURE_SCHED(exec_ctx, closure, GRPC_ERROR_REF(original_error));
+      break;
+    } else {
+      if (gpr_atm_full_cas(&call_combiner->cancel_state, original_state,
+                           (gpr_atm)closure)) {
+        break;
+      }
+    }
+    // cas failed, try again.
+  }
+}
+
+void grpc_call_combiner_cancel(grpc_exec_ctx* exec_ctx,
+                               grpc_call_combiner* call_combiner,
+                               grpc_error* error) {
+  while (true) {
+    gpr_atm original_state = gpr_atm_acq_load(&call_combiner->cancel_state);
+    grpc_error* original_error = decode_cancel_state_error(original_state);
+    if (original_error != GRPC_ERROR_NONE) {
+      GRPC_ERROR_UNREF(error);
+      break;
+    }
+    if (gpr_atm_full_cas(&call_combiner->cancel_state, original_state,
+                         encode_cancel_state_error(error))) {
+      if (original_state != 0) {
+        grpc_closure* notify_on_cancel = (grpc_closure*)original_state;
+        if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+          gpr_log(GPR_DEBUG,
+                  "call_combiner=%p: scheduling notify_on_cancel callback=%p",
+                  call_combiner, notify_on_cancel);
+        }
+        GRPC_CLOSURE_SCHED(exec_ctx, notify_on_cancel, GRPC_ERROR_REF(error));
+      }
+      break;
+    }
+    // cas failed, try again.
+  }
+}
diff --git a/src/core/lib/iomgr/call_combiner.h b/src/core/lib/iomgr/call_combiner.h
new file mode 100644
index 0000000000..621e2c3669
--- /dev/null
+++ b/src/core/lib/iomgr/call_combiner.h
@@ -0,0 +1,104 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#ifndef GRPC_CORE_LIB_IOMGR_CALL_COMBINER_H
+#define GRPC_CORE_LIB_IOMGR_CALL_COMBINER_H
+
+#include <stddef.h>
+
+#include <grpc/support/atm.h>
+
+#include "src/core/lib/iomgr/closure.h"
+#include "src/core/lib/iomgr/exec_ctx.h"
+#include "src/core/lib/support/mpscq.h"
+
+// A simple, lock-free mechanism for serializing activity related to a
+// single call.  This is similar to a combiner but is more lightweight.
+//
+// It requires the callback (or, in the common case where the callback
+// actually kicks off a chain of callbacks, the last callback in that
+// chain) to explicitly indicate (by calling GRPC_CALL_COMBINER_STOP())
+// when it is done with the action that was kicked off by the original
+// callback.
+
+extern grpc_tracer_flag grpc_call_combiner_trace;
+
+typedef struct {
+  gpr_atm size;  // size_t, num closures in queue or currently executing
+  gpr_mpscq queue;
+  // Either 0 (if not cancelled and no cancellation closure set),
+  // a grpc_closure* (if the lowest bit is 0),
+  // or a grpc_error* (if the lowest bit is 1).
+  gpr_atm cancel_state;
+} grpc_call_combiner;
+
+// Assumes memory was initialized to zero.
+void grpc_call_combiner_init(grpc_call_combiner* call_combiner);
+
+void grpc_call_combiner_destroy(grpc_call_combiner* call_combiner);
+
+#ifndef NDEBUG
+#define GRPC_CALL_COMBINER_START(exec_ctx, call_combiner, closure, error,   \
+                                 reason)                                    \
+  grpc_call_combiner_start((exec_ctx), (call_combiner), (closure), (error), \
+                           __FILE__, __LINE__, (reason))
+#define GRPC_CALL_COMBINER_STOP(exec_ctx, call_combiner, reason)           \
+  grpc_call_combiner_stop((exec_ctx), (call_combiner), __FILE__, __LINE__, \
+                          (reason))
+/// Starts processing \a closure on \a call_combiner.
+void grpc_call_combiner_start(grpc_exec_ctx* exec_ctx,
+                              grpc_call_combiner* call_combiner,
+                              grpc_closure* closure, grpc_error* error,
+                              const char* file, int line, const char* reason);
+/// Yields the call combiner to the next closure in the queue, if any.
+void grpc_call_combiner_stop(grpc_exec_ctx* exec_ctx,
+                             grpc_call_combiner* call_combiner,
+                             const char* file, int line, const char* reason);
+#else
+#define GRPC_CALL_COMBINER_START(exec_ctx, call_combiner, closure, error,   \
+                                 reason)                                    \
+  grpc_call_combiner_start((exec_ctx), (call_combiner), (closure), (error), \
+                           (reason))
+#define GRPC_CALL_COMBINER_STOP(exec_ctx, call_combiner, reason) \
+  grpc_call_combiner_stop((exec_ctx), (call_combiner), (reason))
+/// Starts processing \a closure on \a call_combiner.
+void grpc_call_combiner_start(grpc_exec_ctx* exec_ctx,
+                              grpc_call_combiner* call_combiner,
+                              grpc_closure* closure, grpc_error* error,
+                              const char* reason);
+/// Yields the call combiner to the next closure in the queue, if any.
+void grpc_call_combiner_stop(grpc_exec_ctx* exec_ctx,
+                             grpc_call_combiner* call_combiner,
+                             const char* reason);
+#endif
+
+/// Tells \a call_combiner to invoke \a closure when
+/// grpc_call_combiner_cancel() is called.  If grpc_call_combiner_cancel()
+/// was previously called, \a closure will be invoked immediately.
+/// If \a closure is NULL, then no closure will be invoked on
+/// cancellation; this effectively unregisters the previously set closure.
+void grpc_call_combiner_set_notify_on_cancel(grpc_exec_ctx* exec_ctx,
+                                             grpc_call_combiner* call_combiner,
+                                             grpc_closure* closure);
+
+/// Indicates that the call has been cancelled.
+void grpc_call_combiner_cancel(grpc_exec_ctx* exec_ctx,
+                               grpc_call_combiner* call_combiner,
+                               grpc_error* error);
+
+#endif /* GRPC_CORE_LIB_IOMGR_CALL_COMBINER_H */
diff --git a/src/core/lib/security/transport/client_auth_filter.c b/src/core/lib/security/transport/client_auth_filter.c
index 531a88434f..e3f0163a6c 100644
--- a/src/core/lib/security/transport/client_auth_filter.c
+++ b/src/core/lib/security/transport/client_auth_filter.c
@@ -39,6 +39,7 @@
 
 /* We can have a per-call credentials. */
 typedef struct {
+  grpc_call_combiner *call_combiner;
   grpc_call_credentials *creds;
   bool have_host;
   bool have_method;
@@ -49,17 +50,11 @@ typedef struct {
      pollset_set so that work can progress when this call wants work to progress
   */
   grpc_polling_entity *pollent;
-  gpr_atm security_context_set;
-  gpr_mu security_context_mu;
   grpc_credentials_mdelem_array md_array;
   grpc_linked_mdelem md_links[MAX_CREDENTIALS_METADATA_COUNT];
   grpc_auth_metadata_context auth_md_context;
-  grpc_closure closure;
-  // Either 0 (no cancellation and no async operation in flight),
-  // a grpc_closure* (if the lowest bit is 0),
-  // or a grpc_error* (if the lowest bit is 1).
-  gpr_atm cancellation_state;
-  grpc_closure cancel_closure;
+  grpc_closure async_cancel_closure;
+  grpc_closure async_result_closure;
 } call_data;
 
 /* We can have a per-channel credentials. */
@@ -68,43 +63,6 @@ typedef struct {
   grpc_auth_context *auth_context;
 } channel_data;
 
-static void decode_cancel_state(gpr_atm cancel_state, grpc_closure **func,
-                                grpc_error **error) {
-  // If the lowest bit is 1, the value is a grpc_error*.
-  // Otherwise, if non-zdero, the value is a grpc_closure*.
-  if (cancel_state & 1) {
-    *error = (grpc_error *)(cancel_state & ~(gpr_atm)1);
-  } else if (cancel_state != 0) {
-    *func = (grpc_closure *)cancel_state;
-  }
-}
-
-static gpr_atm encode_cancel_state_error(grpc_error *error) {
-  // Set the lowest bit to 1 to indicate that it's an error.
-  return (gpr_atm)1 | (gpr_atm)error;
-}
-
-// Returns an error if the call has been cancelled.  Otherwise, sets the
-// cancellation function to be called upon cancellation.
-static grpc_error *set_cancel_func(grpc_call_element *elem,
-                                   grpc_iomgr_cb_func func) {
-  call_data *calld = (call_data *)elem->call_data;
-  // Decode original state.
-  gpr_atm original_state = gpr_atm_acq_load(&calld->cancellation_state);
-  grpc_error *original_error = GRPC_ERROR_NONE;
-  grpc_closure *original_func = NULL;
-  decode_cancel_state(original_state, &original_func, &original_error);
-  // If error is set, return it.
-  if (original_error != GRPC_ERROR_NONE) return GRPC_ERROR_REF(original_error);
-  // Otherwise, store func.
-  GRPC_CLOSURE_INIT(&calld->cancel_closure, func, elem,
-                    grpc_schedule_on_exec_ctx);
-  GPR_ASSERT(((gpr_atm)&calld->cancel_closure & (gpr_atm)1) == 0);
-  gpr_atm_rel_store(&calld->cancellation_state,
-                    (gpr_atm)&calld->cancel_closure);
-  return GRPC_ERROR_NONE;
-}
-
 static void reset_auth_metadata_context(
     grpc_auth_metadata_context *auth_md_context) {
   if (auth_md_context->service_url != NULL) {
@@ -135,6 +93,7 @@ static void on_credentials_metadata(grpc_exec_ctx *exec_ctx, void *arg,
   grpc_transport_stream_op_batch *batch = (grpc_transport_stream_op_batch *)arg;
   grpc_call_element *elem = batch->handler_private.extra_arg;
   call_data *calld = elem->call_data;
+  grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner, NULL);
   reset_auth_metadata_context(&calld->auth_md_context);
   grpc_error *error = GRPC_ERROR_REF(input_error);
   if (error == GRPC_ERROR_NONE) {
@@ -153,7 +112,8 @@ static void on_credentials_metadata(grpc_exec_ctx *exec_ctx, void *arg,
   } else {
     error = grpc_error_set_int(error, GRPC_ERROR_INT_GRPC_STATUS,
                                GRPC_STATUS_UNAUTHENTICATED);
-    grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch, error);
+    grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch, error,
+                                                       calld->call_combiner);
   }
 }
 
@@ -223,7 +183,8 @@ static void send_security_metadata(grpc_exec_ctx *exec_ctx,
           grpc_error_set_int(
               GRPC_ERROR_CREATE_FROM_STATIC_STRING(
                   "Incompatible credentials set on channel and call."),
-              GRPC_ERROR_INT_GRPC_STATUS, GRPC_STATUS_UNAUTHENTICATED));
+              GRPC_ERROR_INT_GRPC_STATUS, GRPC_STATUS_UNAUTHENTICATED),
+          calld->call_combiner);
       return;
     }
   } else {
@@ -234,22 +195,23 @@ static void send_security_metadata(grpc_exec_ctx *exec_ctx,
   build_auth_metadata_context(&chand->security_connector->base,
                               chand->auth_context, calld);
 
-  grpc_error *cancel_error = set_cancel_func(elem, cancel_get_request_metadata);
-  if (cancel_error != GRPC_ERROR_NONE) {
-    grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch,
-                                                       cancel_error);
-    return;
-  }
   GPR_ASSERT(calld->pollent != NULL);
-  GRPC_CLOSURE_INIT(&calld->closure, on_credentials_metadata, batch,
-                    grpc_schedule_on_exec_ctx);
+
+  GRPC_CLOSURE_INIT(&calld->async_result_closure, on_credentials_metadata,
+                    batch, grpc_schedule_on_exec_ctx);
   grpc_error *error = GRPC_ERROR_NONE;
   if (grpc_call_credentials_get_request_metadata(
           exec_ctx, calld->creds, calld->pollent, calld->auth_md_context,
-          &calld->md_array, &calld->closure, &error)) {
+          &calld->md_array, &calld->async_result_closure, &error)) {
     // Synchronous return; invoke on_credentials_metadata() directly.
     on_credentials_metadata(exec_ctx, batch, error);
     GRPC_ERROR_UNREF(error);
+  } else {
+    // Async return; register cancellation closure with call combiner.
+    GRPC_CLOSURE_INIT(&calld->async_cancel_closure, cancel_get_request_metadata,
+                      elem, grpc_schedule_on_exec_ctx);
+    grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner,
+                                            &calld->async_cancel_closure);
   }
 }
 
@@ -258,7 +220,7 @@ static void on_host_checked(grpc_exec_ctx *exec_ctx, void *arg,
   grpc_transport_stream_op_batch *batch = (grpc_transport_stream_op_batch *)arg;
   grpc_call_element *elem = batch->handler_private.extra_arg;
   call_data *calld = elem->call_data;
-
+  grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner, NULL);
   if (error == GRPC_ERROR_NONE) {
     send_security_metadata(exec_ctx, elem, batch);
   } else {
@@ -271,7 +233,8 @@ static void on_host_checked(grpc_exec_ctx *exec_ctx, void *arg,
         exec_ctx, batch,
         grpc_error_set_int(GRPC_ERROR_CREATE_FROM_COPIED_STRING(error_msg),
                            GRPC_ERROR_INT_GRPC_STATUS,
-                           GRPC_STATUS_UNAUTHENTICATED));
+                           GRPC_STATUS_UNAUTHENTICATED),
+        calld->call_combiner);
     gpr_free(error_msg);
   }
 }
@@ -282,7 +245,7 @@ static void cancel_check_call_host(grpc_exec_ctx *exec_ctx, void *arg,
   call_data *calld = (call_data *)elem->call_data;
   channel_data *chand = (channel_data *)elem->channel_data;
   grpc_channel_security_connector_cancel_check_call_host(
-      exec_ctx, chand->security_connector, &calld->closure,
+      exec_ctx, chand->security_connector, &calld->async_result_closure,
       GRPC_ERROR_REF(error));
 }
 
@@ -295,52 +258,19 @@ static void auth_start_transport_stream_op_batch(
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
 
-  if (batch->cancel_stream) {
-    while (true) {
-      // Decode the original cancellation state.
-      gpr_atm original_state = gpr_atm_acq_load(&calld->cancellation_state);
-      grpc_error *cancel_error = GRPC_ERROR_NONE;
-      grpc_closure *func = NULL;
-      decode_cancel_state(original_state, &func, &cancel_error);
-      // If we had already set a cancellation error, there's nothing
-      // more to do.
-      if (cancel_error != GRPC_ERROR_NONE) break;
-      // If there's a cancel func, call it.
-      // Note that even if the cancel func has been changed by some
-      // other thread between when we decoded it and now, it will just
-      // be a no-op.
-      cancel_error = GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error);
-      if (func != NULL) {
-        GRPC_CLOSURE_SCHED(exec_ctx, func, GRPC_ERROR_REF(cancel_error));
-      }
-      // Encode the new error into cancellation state.
-      if (gpr_atm_full_cas(&calld->cancellation_state, original_state,
-                           encode_cancel_state_error(cancel_error))) {
-        break;  // Success.
-      }
-      // The cas failed, so try again.
-    }
-  } else {
-    /* double checked lock over security context to ensure it's set once */
-    if (gpr_atm_acq_load(&calld->security_context_set) == 0) {
-      gpr_mu_lock(&calld->security_context_mu);
-      if (gpr_atm_acq_load(&calld->security_context_set) == 0) {
-        GPR_ASSERT(batch->payload->context != NULL);
-        if (batch->payload->context[GRPC_CONTEXT_SECURITY].value == NULL) {
-          batch->payload->context[GRPC_CONTEXT_SECURITY].value =
-              grpc_client_security_context_create();
-          batch->payload->context[GRPC_CONTEXT_SECURITY].destroy =
-              grpc_client_security_context_destroy;
-        }
-        grpc_client_security_context *sec_ctx =
-            batch->payload->context[GRPC_CONTEXT_SECURITY].value;
-        GRPC_AUTH_CONTEXT_UNREF(sec_ctx->auth_context, "client auth filter");
-        sec_ctx->auth_context =
-            GRPC_AUTH_CONTEXT_REF(chand->auth_context, "client_auth_filter");
-        gpr_atm_rel_store(&calld->security_context_set, 1);
-      }
-      gpr_mu_unlock(&calld->security_context_mu);
+  if (!batch->cancel_stream) {
+    GPR_ASSERT(batch->payload->context != NULL);
+    if (batch->payload->context[GRPC_CONTEXT_SECURITY].value == NULL) {
+      batch->payload->context[GRPC_CONTEXT_SECURITY].value =
+          grpc_client_security_context_create();
+      batch->payload->context[GRPC_CONTEXT_SECURITY].destroy =
+          grpc_client_security_context_destroy;
     }
+    grpc_client_security_context *sec_ctx =
+        batch->payload->context[GRPC_CONTEXT_SECURITY].value;
+    GRPC_AUTH_CONTEXT_UNREF(sec_ctx->auth_context, "client auth filter");
+    sec_ctx->auth_context =
+        GRPC_AUTH_CONTEXT_REF(chand->auth_context, "client_auth_filter");
   }
 
   if (batch->send_initial_metadata) {
@@ -365,26 +295,25 @@ static void auth_start_transport_stream_op_batch(
       }
     }
     if (calld->have_host) {
-      grpc_error *cancel_error = set_cancel_func(elem, cancel_check_call_host);
-      if (cancel_error != GRPC_ERROR_NONE) {
-        grpc_transport_stream_op_batch_finish_with_failure(exec_ctx, batch,
-                                                           cancel_error);
+      batch->handler_private.extra_arg = elem;
+      GRPC_CLOSURE_INIT(&calld->async_result_closure, on_host_checked, batch,
+                        grpc_schedule_on_exec_ctx);
+      char *call_host = grpc_slice_to_c_string(calld->host);
+      grpc_error *error = GRPC_ERROR_NONE;
+      if (grpc_channel_security_connector_check_call_host(
+              exec_ctx, chand->security_connector, call_host,
+              chand->auth_context, &calld->async_result_closure, &error)) {
+        // Synchronous return; invoke on_host_checked() directly.
+        on_host_checked(exec_ctx, batch, error);
+        GRPC_ERROR_UNREF(error);
       } else {
-        char *call_host = grpc_slice_to_c_string(calld->host);
-        batch->handler_private.extra_arg = elem;
-        grpc_error *error = GRPC_ERROR_NONE;
-        if (grpc_channel_security_connector_check_call_host(
-                exec_ctx, chand->security_connector, call_host,
-                chand->auth_context,
-                GRPC_CLOSURE_INIT(&calld->closure, on_host_checked, batch,
-                                  grpc_schedule_on_exec_ctx),
-                &error)) {
-          // Synchronous return; invoke on_host_checked() directly.
-          on_host_checked(exec_ctx, batch, error);
-          GRPC_ERROR_UNREF(error);
-        }
-        gpr_free(call_host);
+        // Async return; register cancellation closure with call combiner.
+        GRPC_CLOSURE_INIT(&calld->async_cancel_closure, cancel_check_call_host,
+                          elem, grpc_schedule_on_exec_ctx);
+        grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner,
+                                                &calld->async_cancel_closure);
       }
+      gpr_free(call_host);
       GPR_TIMER_END("auth_start_transport_stream_op_batch", 0);
       return; /* early exit */
     }
@@ -400,8 +329,7 @@ static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem,
                                   const grpc_call_element_args *args) {
   call_data *calld = elem->call_data;
-  memset(calld, 0, sizeof(*calld));
-  gpr_mu_init(&calld->security_context_mu);
+  calld->call_combiner = args->call_combiner;
   return GRPC_ERROR_NONE;
 }
 
@@ -426,12 +354,6 @@ static void destroy_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
     grpc_slice_unref_internal(exec_ctx, calld->method);
   }
   reset_auth_metadata_context(&calld->auth_md_context);
-  gpr_mu_destroy(&calld->security_context_mu);
-  gpr_atm cancel_state = gpr_atm_acq_load(&calld->cancellation_state);
-  grpc_error *cancel_error = GRPC_ERROR_NONE;
-  grpc_closure *cancel_func = NULL;
-  decode_cancel_state(cancel_state, &cancel_func, &cancel_error);
-  GRPC_ERROR_UNREF(cancel_error);
 }
 
 /* Constructor for channel_data */
@@ -490,6 +412,5 @@ const grpc_channel_filter grpc_client_auth_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "client-auth"};
diff --git a/src/core/lib/security/transport/server_auth_filter.c b/src/core/lib/security/transport/server_auth_filter.c
index 9bf3f0ca0f..b721ce4a22 100644
--- a/src/core/lib/security/transport/server_auth_filter.c
+++ b/src/core/lib/security/transport/server_auth_filter.c
@@ -26,7 +26,15 @@
 #include "src/core/lib/security/transport/auth_filters.h"
 #include "src/core/lib/slice/slice_internal.h"
 
+typedef enum {
+  STATE_INIT = 0,
+  STATE_DONE,
+  STATE_CANCELLED,
+} async_state;
+
 typedef struct call_data {
+  grpc_call_combiner *call_combiner;
+  grpc_call_stack *owning_call;
   grpc_transport_stream_op_batch *recv_initial_metadata_batch;
   grpc_closure *original_recv_initial_metadata_ready;
   grpc_closure recv_initial_metadata_ready;
@@ -34,6 +42,8 @@ typedef struct call_data {
   const grpc_metadata *consumed_md;
   size_t num_consumed_md;
   grpc_auth_context *auth_context;
+  grpc_closure cancel_closure;
+  gpr_atm state;  // async_state
 } call_data;
 
 typedef struct channel_data {
@@ -78,54 +88,92 @@ static grpc_filtered_mdelem remove_consumed_md(grpc_exec_ctx *exec_ctx,
   return GRPC_FILTERED_MDELEM(md);
 }
 
-/* called from application code */
-static void on_md_processing_done(
-    void *user_data, const grpc_metadata *consumed_md, size_t num_consumed_md,
-    const grpc_metadata *response_md, size_t num_response_md,
-    grpc_status_code status, const char *error_details) {
-  grpc_call_element *elem = user_data;
+static void on_md_processing_done_inner(grpc_exec_ctx *exec_ctx,
+                                        grpc_call_element *elem,
+                                        const grpc_metadata *consumed_md,
+                                        size_t num_consumed_md,
+                                        const grpc_metadata *response_md,
+                                        size_t num_response_md,
+                                        grpc_error *error) {
   call_data *calld = elem->call_data;
   grpc_transport_stream_op_batch *batch = calld->recv_initial_metadata_batch;
-  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
+  grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner, NULL);
   /* TODO(jboeuf): Implement support for response_md. */
   if (response_md != NULL && num_response_md > 0) {
     gpr_log(GPR_INFO,
             "response_md in auth metadata processing not supported for now. "
             "Ignoring...");
   }
-  grpc_error *error = GRPC_ERROR_NONE;
-  if (status == GRPC_STATUS_OK) {
+  if (error == GRPC_ERROR_NONE) {
     calld->consumed_md = consumed_md;
     calld->num_consumed_md = num_consumed_md;
     error = grpc_metadata_batch_filter(
-        &exec_ctx, batch->payload->recv_initial_metadata.recv_initial_metadata,
+        exec_ctx, batch->payload->recv_initial_metadata.recv_initial_metadata,
         remove_consumed_md, elem, "Response metadata filtering error");
-  } else {
-    if (error_details == NULL) {
-      error_details = "Authentication metadata processing failed.";
+  }
+  GRPC_CLOSURE_SCHED(exec_ctx, calld->original_recv_initial_metadata_ready,
+                     error);
+}
+
+// Called from application code.
+static void on_md_processing_done(
+    void *user_data, const grpc_metadata *consumed_md, size_t num_consumed_md,
+    const grpc_metadata *response_md, size_t num_response_md,
+    grpc_status_code status, const char *error_details) {
+  grpc_call_element *elem = user_data;
+  call_data *calld = elem->call_data;
+  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
+  // If the call was not cancelled while we were in flight, process the result.
+  if (gpr_atm_full_cas(&calld->state, (gpr_atm)STATE_INIT,
+                       (gpr_atm)STATE_DONE)) {
+    grpc_error *error = GRPC_ERROR_NONE;
+    if (status != GRPC_STATUS_OK) {
+      if (error_details == NULL) {
+        error_details = "Authentication metadata processing failed.";
+      }
+      error = grpc_error_set_int(
+          GRPC_ERROR_CREATE_FROM_COPIED_STRING(error_details),
+          GRPC_ERROR_INT_GRPC_STATUS, status);
     }
-    error =
-        grpc_error_set_int(GRPC_ERROR_CREATE_FROM_COPIED_STRING(error_details),
-                           GRPC_ERROR_INT_GRPC_STATUS, status);
+    on_md_processing_done_inner(&exec_ctx, elem, consumed_md, num_consumed_md,
+                                response_md, num_response_md, error);
   }
+  // Clean up.
   for (size_t i = 0; i < calld->md.count; i++) {
     grpc_slice_unref_internal(&exec_ctx, calld->md.metadata[i].key);
     grpc_slice_unref_internal(&exec_ctx, calld->md.metadata[i].value);
   }
   grpc_metadata_array_destroy(&calld->md);
-  GRPC_CLOSURE_SCHED(&exec_ctx, calld->original_recv_initial_metadata_ready,
-                     error);
+  GRPC_CALL_STACK_UNREF(&exec_ctx, calld->owning_call, "server_auth_metadata");
   grpc_exec_ctx_finish(&exec_ctx);
 }
 
+static void cancel_call(grpc_exec_ctx *exec_ctx, void *arg, grpc_error *error) {
+  grpc_call_element *elem = (grpc_call_element *)arg;
+  call_data *calld = elem->call_data;
+  // If the result was not already processed, invoke the callback now.
+  if (gpr_atm_full_cas(&calld->state, (gpr_atm)STATE_INIT,
+                       (gpr_atm)STATE_CANCELLED)) {
+    on_md_processing_done_inner(exec_ctx, elem, NULL, 0, NULL, 0,
+                                GRPC_ERROR_REF(error));
+  }
+}
+
 static void recv_initial_metadata_ready(grpc_exec_ctx *exec_ctx, void *arg,
                                         grpc_error *error) {
-  grpc_call_element *elem = arg;
+  grpc_call_element *elem = (grpc_call_element *)arg;
   channel_data *chand = elem->channel_data;
   call_data *calld = elem->call_data;
   grpc_transport_stream_op_batch *batch = calld->recv_initial_metadata_batch;
   if (error == GRPC_ERROR_NONE) {
     if (chand->creds != NULL && chand->creds->processor.process != NULL) {
+      // We're calling out to the application, so we need to make sure
+      // to drop the call combiner early if we get cancelled.
+      GRPC_CLOSURE_INIT(&calld->cancel_closure, cancel_call, elem,
+                        grpc_schedule_on_exec_ctx);
+      grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner,
+                                              &calld->cancel_closure);
+      GRPC_CALL_STACK_REF(calld->owning_call, "server_auth_metadata");
       calld->md = metadata_batch_to_md_array(
           batch->payload->recv_initial_metadata.recv_initial_metadata);
       chand->creds->processor.process(
@@ -159,6 +207,8 @@ static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
                                   const grpc_call_element_args *args) {
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
+  calld->call_combiner = args->call_combiner;
+  calld->owning_call = args->call_stack;
   GRPC_CLOSURE_INIT(&calld->recv_initial_metadata_ready,
                     recv_initial_metadata_ready, elem,
                     grpc_schedule_on_exec_ctx);
@@ -218,6 +268,5 @@ const grpc_channel_filter grpc_server_auth_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "server-auth"};
diff --git a/src/core/lib/surface/call.c b/src/core/lib/surface/call.c
index e68c201134..03eaaf99ac 100644
--- a/src/core/lib/surface/call.c
+++ b/src/core/lib/surface/call.c
@@ -122,6 +122,7 @@ typedef struct batch_control {
       bool is_closure;
     } notify_tag;
   } completion_data;
+  grpc_closure start_batch;
   grpc_closure finish_batch;
   gpr_refcount steps_to_complete;
 
@@ -151,6 +152,7 @@ typedef struct {
 struct grpc_call {
   gpr_refcount ext_ref;
   gpr_arena *arena;
+  grpc_call_combiner call_combiner;
   grpc_completion_queue *cq;
   grpc_polling_entity pollent;
   grpc_channel *channel;
@@ -184,6 +186,11 @@ struct grpc_call {
      Element 0 is initial metadata, element 1 is trailing metadata. */
   grpc_metadata_array *buffered_metadata[2];
 
+  grpc_metadata compression_md;
+
+  // A char* indicating the peer name.
+  gpr_atm peer_string;
+
   /* Packed received call statuses from various sources */
   gpr_atm status[STATUS_SOURCE_COUNT];
 
@@ -262,8 +269,9 @@ grpc_tracer_flag grpc_compression_trace =
 #define CALL_FROM_TOP_ELEM(top_elem) \
   CALL_FROM_CALL_STACK(grpc_call_stack_from_top_element(top_elem))
 
-static void execute_op(grpc_exec_ctx *exec_ctx, grpc_call *call,
-                       grpc_transport_stream_op_batch *op);
+static void execute_batch(grpc_exec_ctx *exec_ctx, grpc_call *call,
+                          grpc_transport_stream_op_batch *op,
+                          grpc_closure *start_batch_closure);
 static void cancel_with_status(grpc_exec_ctx *exec_ctx, grpc_call *c,
                                status_source source, grpc_status_code status,
                                const char *description);
@@ -328,6 +336,7 @@ grpc_error *grpc_call_create(grpc_exec_ctx *exec_ctx,
                          sizeof(grpc_call) + channel_stack->call_stack_size);
   gpr_ref_init(&call->ext_ref, 1);
   call->arena = arena;
+  grpc_call_combiner_init(&call->call_combiner);
   *out_call = call;
   call->channel = args->channel;
   call->cq = args->cq;
@@ -436,7 +445,8 @@ grpc_error *grpc_call_create(grpc_exec_ctx *exec_ctx,
       .path = path,
       .start_time = call->start_time,
       .deadline = send_deadline,
-      .arena = call->arena};
+      .arena = call->arena,
+      .call_combiner = &call->call_combiner};
   add_init_error(&error, grpc_call_stack_init(exec_ctx, channel_stack, 1,
                                               destroy_call, call, &call_args));
   if (error != GRPC_ERROR_NONE) {
@@ -503,6 +513,8 @@ static void release_call(grpc_exec_ctx *exec_ctx, void *call,
                          grpc_error *error) {
   grpc_call *c = call;
   grpc_channel *channel = c->channel;
+  grpc_call_combiner_destroy(&c->call_combiner);
+  gpr_free((char *)c->peer_string);
   grpc_channel_update_call_size_estimate(channel, gpr_arena_destroy(c->arena));
   GRPC_CHANNEL_INTERNAL_UNREF(exec_ctx, channel, "call");
 }
@@ -602,30 +614,37 @@ grpc_call_error grpc_call_cancel(grpc_call *call, void *reserved) {
   return GRPC_CALL_OK;
 }
 
-static void execute_op(grpc_exec_ctx *exec_ctx, grpc_call *call,
-                       grpc_transport_stream_op_batch *op) {
-  grpc_call_element *elem;
-
-  GPR_TIMER_BEGIN("execute_op", 0);
-  elem = CALL_ELEM_FROM_CALL(call, 0);
-  elem->filter->start_transport_stream_op_batch(exec_ctx, elem, op);
-  GPR_TIMER_END("execute_op", 0);
+// This is called via the call combiner to start sending a batch down
+// the filter stack.
+static void execute_batch_in_call_combiner(grpc_exec_ctx *exec_ctx, void *arg,
+                                           grpc_error *ignored) {
+  grpc_transport_stream_op_batch *batch = arg;
+  grpc_call *call = batch->handler_private.extra_arg;
+  GPR_TIMER_BEGIN("execute_batch", 0);
+  grpc_call_element *elem = CALL_ELEM_FROM_CALL(call, 0);
+  GRPC_CALL_LOG_OP(GPR_INFO, elem, batch);
+  elem->filter->start_transport_stream_op_batch(exec_ctx, elem, batch);
+  GPR_TIMER_END("execute_batch", 0);
+}
+
+// start_batch_closure points to a caller-allocated closure to be used
+// for entering the call combiner.
+static void execute_batch(grpc_exec_ctx *exec_ctx, grpc_call *call,
+                          grpc_transport_stream_op_batch *batch,
+                          grpc_closure *start_batch_closure) {
+  batch->handler_private.extra_arg = call;
+  GRPC_CLOSURE_INIT(start_batch_closure, execute_batch_in_call_combiner, batch,
+                    grpc_schedule_on_exec_ctx);
+  GRPC_CALL_COMBINER_START(exec_ctx, &call->call_combiner, start_batch_closure,
+                           GRPC_ERROR_NONE, "executing batch");
 }
 
 char *grpc_call_get_peer(grpc_call *call) {
-  grpc_call_element *elem = CALL_ELEM_FROM_CALL(call, 0);
-  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
-  char *result;
-  GRPC_API_TRACE("grpc_call_get_peer(%p)", 1, (call));
-  result = elem->filter->get_peer(&exec_ctx, elem);
-  if (result == NULL) {
-    result = grpc_channel_get_target(call->channel);
-  }
-  if (result == NULL) {
-    result = gpr_strdup("unknown");
-  }
-  grpc_exec_ctx_finish(&exec_ctx);
-  return result;
+  char *peer_string = (char *)gpr_atm_acq_load(&call->peer_string);
+  if (peer_string != NULL) return gpr_strdup(peer_string);
+  peer_string = grpc_channel_get_target(call->channel);
+  if (peer_string != NULL) return peer_string;
+  return gpr_strdup("unknown");
 }
 
 grpc_call *grpc_call_from_top_element(grpc_call_element *elem) {
@@ -652,20 +671,41 @@ grpc_call_error grpc_call_cancel_with_status(grpc_call *c,
   return GRPC_CALL_OK;
 }
 
-static void done_termination(grpc_exec_ctx *exec_ctx, void *call,
+typedef struct {
+  grpc_call *call;
+  grpc_closure start_batch;
+  grpc_closure finish_batch;
+} cancel_state;
+
+// The on_complete callback used when sending a cancel_stream batch down
+// the filter stack.  Yields the call combiner when the batch is done.
+static void done_termination(grpc_exec_ctx *exec_ctx, void *arg,
                              grpc_error *error) {
-  GRPC_CALL_INTERNAL_UNREF(exec_ctx, call, "termination");
+  cancel_state *state = (cancel_state *)arg;
+  GRPC_CALL_COMBINER_STOP(exec_ctx, &state->call->call_combiner,
+                          "on_complete for cancel_stream op");
+  GRPC_CALL_INTERNAL_UNREF(exec_ctx, state->call, "termination");
+  gpr_free(state);
 }
 
 static void cancel_with_error(grpc_exec_ctx *exec_ctx, grpc_call *c,
                               status_source source, grpc_error *error) {
   GRPC_CALL_INTERNAL_REF(c, "termination");
+  // Inform the call combiner of the cancellation, so that it can cancel
+  // any in-flight asynchronous actions that may be holding the call
+  // combiner.  This ensures that the cancel_stream batch can be sent
+  // down the filter stack in a timely manner.
+  grpc_call_combiner_cancel(exec_ctx, &c->call_combiner, GRPC_ERROR_REF(error));
   set_status_from_error(exec_ctx, c, source, GRPC_ERROR_REF(error));
-  grpc_transport_stream_op_batch *op = grpc_make_transport_stream_op(
-      GRPC_CLOSURE_CREATE(done_termination, c, grpc_schedule_on_exec_ctx));
+  cancel_state *state = (cancel_state *)gpr_malloc(sizeof(*state));
+  state->call = c;
+  GRPC_CLOSURE_INIT(&state->finish_batch, done_termination, state,
+                    grpc_schedule_on_exec_ctx);
+  grpc_transport_stream_op_batch *op =
+      grpc_make_transport_stream_op(&state->finish_batch);
   op->cancel_stream = true;
   op->payload->cancel_stream.cancel_error = error;
-  execute_op(exec_ctx, c, op);
+  execute_batch(exec_ctx, c, op, &state->start_batch);
 }
 
 static grpc_error *error_from_status(grpc_status_code status,
@@ -1431,6 +1471,18 @@ static void receiving_stream_ready(grpc_exec_ctx *exec_ctx, void *bctlp,
   }
 }
 
+// The recv_message_ready callback used when sending a batch containing
+// a recv_message op down the filter stack.  Yields the call combiner
+// before processing the received message.
+static void receiving_stream_ready_in_call_combiner(grpc_exec_ctx *exec_ctx,
+                                                    void *bctlp,
+                                                    grpc_error *error) {
+  batch_control *bctl = bctlp;
+  grpc_call *call = bctl->call;
+  GRPC_CALL_COMBINER_STOP(exec_ctx, &call->call_combiner, "recv_message_ready");
+  receiving_stream_ready(exec_ctx, bctlp, error);
+}
+
 static void validate_filtered_metadata(grpc_exec_ctx *exec_ctx,
                                        batch_control *bctl) {
   grpc_call *call = bctl->call;
@@ -1537,6 +1589,9 @@ static void receiving_initial_metadata_ready(grpc_exec_ctx *exec_ctx,
   batch_control *bctl = bctlp;
   grpc_call *call = bctl->call;
 
+  GRPC_CALL_COMBINER_STOP(exec_ctx, &call->call_combiner,
+                          "recv_initial_metadata_ready");
+
   add_batch_error(exec_ctx, bctl, GRPC_ERROR_REF(error), false);
   if (error == GRPC_ERROR_NONE) {
     grpc_metadata_batch *md =
@@ -1590,7 +1645,8 @@ static void receiving_initial_metadata_ready(grpc_exec_ctx *exec_ctx,
 static void finish_batch(grpc_exec_ctx *exec_ctx, void *bctlp,
                          grpc_error *error) {
   batch_control *bctl = bctlp;
-
+  grpc_call *call = bctl->call;
+  GRPC_CALL_COMBINER_STOP(exec_ctx, &call->call_combiner, "on_complete");
   add_batch_error(exec_ctx, bctl, GRPC_ERROR_REF(error), false);
   finish_batch_step(exec_ctx, bctl);
 }
@@ -1610,9 +1666,6 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
   int num_completion_callbacks_needed = 1;
   grpc_call_error error = GRPC_CALL_OK;
 
-  // sent_initial_metadata guards against variable reuse.
-  grpc_metadata compression_md;
-
   GPR_TIMER_BEGIN("grpc_call_start_batch", 0);
   GRPC_CALL_LOG_BATCH(GPR_INFO, call, ops, nops, notify_tag);
 
@@ -1660,7 +1713,7 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
           goto done_with_error;
         }
         /* process compression level */
-        memset(&compression_md, 0, sizeof(compression_md));
+        memset(&call->compression_md, 0, sizeof(call->compression_md));
         size_t additional_metadata_count = 0;
         grpc_compression_level effective_compression_level =
             GRPC_COMPRESS_LEVEL_NONE;
@@ -1698,9 +1751,9 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
             const grpc_stream_compression_algorithm calgo =
                 stream_compression_algorithm_for_level_locked(
                     call, effective_stream_compression_level);
-            compression_md.key =
+            call->compression_md.key =
                 GRPC_MDSTR_GRPC_INTERNAL_STREAM_ENCODING_REQUEST;
-            compression_md.value =
+            call->compression_md.value =
                 grpc_stream_compression_algorithm_slice(calgo);
           } else {
             const grpc_compression_algorithm calgo =
@@ -1708,8 +1761,10 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
                     call, effective_compression_level);
             /* the following will be picked up by the compress filter and used
              * as the call's compression algorithm. */
-            compression_md.key = GRPC_MDSTR_GRPC_INTERNAL_ENCODING_REQUEST;
-            compression_md.value = grpc_compression_algorithm_slice(calgo);
+            call->compression_md.key =
+                GRPC_MDSTR_GRPC_INTERNAL_ENCODING_REQUEST;
+            call->compression_md.value =
+                grpc_compression_algorithm_slice(calgo);
             additional_metadata_count++;
           }
         }
@@ -1724,7 +1779,7 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
         if (!prepare_application_metadata(
                 exec_ctx, call, (int)op->data.send_initial_metadata.count,
                 op->data.send_initial_metadata.metadata, 0, call->is_client,
-                &compression_md, (int)additional_metadata_count)) {
+                &call->compression_md, (int)additional_metadata_count)) {
           error = GRPC_CALL_ERROR_INVALID_METADATA;
           goto done_with_error;
         }
@@ -1736,6 +1791,10 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
             &call->metadata_batch[0 /* is_receiving */][0 /* is_trailing */];
         stream_op_payload->send_initial_metadata.send_initial_metadata_flags =
             op->flags;
+        if (call->is_client) {
+          stream_op_payload->send_initial_metadata.peer_string =
+              &call->peer_string;
+        }
         break;
       case GRPC_OP_SEND_MESSAGE:
         if (!are_write_flags_valid(op->flags)) {
@@ -1868,6 +1927,10 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
             &call->metadata_batch[1 /* is_receiving */][0 /* is_trailing */];
         stream_op_payload->recv_initial_metadata.recv_initial_metadata_ready =
             &call->receiving_initial_metadata_ready;
+        if (!call->is_client) {
+          stream_op_payload->recv_initial_metadata.peer_string =
+              &call->peer_string;
+        }
         num_completion_callbacks_needed++;
         break;
       case GRPC_OP_RECV_MESSAGE:
@@ -1884,8 +1947,9 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
         stream_op->recv_message = true;
         call->receiving_buffer = op->data.recv_message.recv_message;
         stream_op_payload->recv_message.recv_message = &call->receiving_stream;
-        GRPC_CLOSURE_INIT(&call->receiving_stream_ready, receiving_stream_ready,
-                          bctl, grpc_schedule_on_exec_ctx);
+        GRPC_CLOSURE_INIT(&call->receiving_stream_ready,
+                          receiving_stream_ready_in_call_combiner, bctl,
+                          grpc_schedule_on_exec_ctx);
         stream_op_payload->recv_message.recv_message_ready =
             &call->receiving_stream_ready;
         num_completion_callbacks_needed++;
@@ -1955,7 +2019,7 @@ static grpc_call_error call_start_batch(grpc_exec_ctx *exec_ctx,
   stream_op->on_complete = &bctl->finish_batch;
   gpr_atm_rel_store(&call->any_ops_sent_atm, 1);
 
-  execute_op(exec_ctx, call, stream_op);
+  execute_batch(exec_ctx, call, stream_op, &bctl->start_batch);
 
 done:
   GPR_TIMER_END("grpc_call_start_batch", 0);
diff --git a/src/core/lib/surface/init.c b/src/core/lib/surface/init.c
index 898476daee..280315036f 100644
--- a/src/core/lib/surface/init.c
+++ b/src/core/lib/surface/init.c
@@ -31,6 +31,7 @@
 #include "src/core/lib/debug/stats.h"
 #include "src/core/lib/debug/trace.h"
 #include "src/core/lib/http/parser.h"
+#include "src/core/lib/iomgr/call_combiner.h"
 #include "src/core/lib/iomgr/combiner.h"
 #include "src/core/lib/iomgr/executor.h"
 #include "src/core/lib/iomgr/iomgr.h"
@@ -129,6 +130,7 @@ void grpc_init(void) {
     grpc_register_tracer(&grpc_trace_channel_stack_builder);
     grpc_register_tracer(&grpc_http1_trace);
     grpc_register_tracer(&grpc_cq_pluck_trace);  // default on
+    grpc_register_tracer(&grpc_call_combiner_trace);
     grpc_register_tracer(&grpc_combiner_trace);
     grpc_register_tracer(&grpc_server_channel_trace);
     grpc_register_tracer(&grpc_bdp_estimator_trace);
diff --git a/src/core/lib/surface/lame_client.cc b/src/core/lib/surface/lame_client.cc
index a0791080a9..6286f9159d 100644
--- a/src/core/lib/surface/lame_client.cc
+++ b/src/core/lib/surface/lame_client.cc
@@ -40,6 +40,7 @@ namespace grpc_core {
 namespace {
 
 struct CallData {
+  grpc_call_combiner *call_combiner;
   grpc_linked_mdelem status;
   grpc_linked_mdelem details;
   grpc_core::atomic<bool> filled_metadata;
@@ -52,14 +53,14 @@ struct ChannelData {
 
 static void fill_metadata(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
                           grpc_metadata_batch *mdb) {
-  CallData *calld = static_cast<CallData *>(elem->call_data);
+  CallData *calld = reinterpret_cast<CallData *>(elem->call_data);
   bool expected = false;
   if (!calld->filled_metadata.compare_exchange_strong(
           expected, true, grpc_core::memory_order_relaxed,
           grpc_core::memory_order_relaxed)) {
     return;
   }
-  ChannelData *chand = static_cast<ChannelData *>(elem->channel_data);
+  ChannelData *chand = reinterpret_cast<ChannelData *>(elem->channel_data);
   char tmp[GPR_LTOA_MIN_BUFSIZE];
   gpr_ltoa(chand->error_code, tmp);
   calld->status.md = grpc_mdelem_from_slices(
@@ -79,6 +80,7 @@ static void fill_metadata(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
 static void lame_start_transport_stream_op_batch(
     grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
     grpc_transport_stream_op_batch *op) {
+  CallData *calld = reinterpret_cast<CallData *>(elem->call_data);
   if (op->recv_initial_metadata) {
     fill_metadata(exec_ctx, elem,
                   op->payload->recv_initial_metadata.recv_initial_metadata);
@@ -87,12 +89,8 @@ static void lame_start_transport_stream_op_batch(
                   op->payload->recv_trailing_metadata.recv_trailing_metadata);
   }
   grpc_transport_stream_op_batch_finish_with_failure(
-      exec_ctx, op,
-      GRPC_ERROR_CREATE_FROM_STATIC_STRING("lame client channel"));
-}
-
-static char *lame_get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  return NULL;
+      exec_ctx, op, GRPC_ERROR_CREATE_FROM_STATIC_STRING("lame client channel"),
+      calld->call_combiner);
 }
 
 static void lame_get_channel_info(grpc_exec_ctx *exec_ctx,
@@ -122,6 +120,8 @@ static void lame_start_transport_op(grpc_exec_ctx *exec_ctx,
 static grpc_error *init_call_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem,
                                   const grpc_call_element_args *args) {
+  CallData *calld = reinterpret_cast<CallData *>(elem->call_data);
+  calld->call_combiner = args->call_combiner;
   return GRPC_ERROR_NONE;
 }
 
@@ -156,7 +156,6 @@ extern "C" const grpc_channel_filter grpc_lame_filter = {
     sizeof(grpc_core::ChannelData),
     grpc_core::init_channel_elem,
     grpc_core::destroy_channel_elem,
-    grpc_core::lame_get_peer,
     grpc_core::lame_get_channel_info,
     "lame-client",
 };
@@ -176,7 +175,7 @@ grpc_channel *grpc_lame_client_channel_create(const char *target,
       "error_message=%s)",
       3, (target, (int)error_code, error_message));
   GPR_ASSERT(elem->filter == &grpc_lame_filter);
-  auto chand = static_cast<grpc_core::ChannelData *>(elem->channel_data);
+  auto chand = reinterpret_cast<grpc_core::ChannelData *>(elem->channel_data);
   chand->error_code = error_code;
   chand->error_message = error_message;
   grpc_exec_ctx_finish(&exec_ctx);
diff --git a/src/core/lib/surface/server.c b/src/core/lib/surface/server.c
index 66dcc299aa..8582d826ca 100644
--- a/src/core/lib/surface/server.c
+++ b/src/core/lib/surface/server.c
@@ -789,7 +789,6 @@ static void server_mutate_op(grpc_call_element *elem,
 static void server_start_transport_stream_op_batch(
     grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
     grpc_transport_stream_op_batch *op) {
-  GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
   server_mutate_op(elem, op);
   grpc_call_next_op(exec_ctx, elem, op);
 }
@@ -962,7 +961,6 @@ const grpc_channel_filter grpc_server_top_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "server",
 };
diff --git a/src/core/lib/transport/transport.c b/src/core/lib/transport/transport.c
index 6c61f4b8d9..650b0559aa 100644
--- a/src/core/lib/transport/transport.c
+++ b/src/core/lib/transport/transport.c
@@ -197,11 +197,6 @@ void grpc_transport_destroy_stream(grpc_exec_ctx *exec_ctx,
                                     then_schedule_closure);
 }
 
-char *grpc_transport_get_peer(grpc_exec_ctx *exec_ctx,
-                              grpc_transport *transport) {
-  return transport->vtable->get_peer(exec_ctx, transport);
-}
-
 grpc_endpoint *grpc_transport_get_endpoint(grpc_exec_ctx *exec_ctx,
                                            grpc_transport *transport) {
   return transport->vtable->get_endpoint(exec_ctx, transport);
@@ -214,24 +209,24 @@ grpc_endpoint *grpc_transport_get_endpoint(grpc_exec_ctx *exec_ctx,
 // is a function that must always unref cancel_error
 // though it lives in lib, it handles transport stream ops sure
 // it's grpc_transport_stream_op_batch_finish_with_failure
-
 void grpc_transport_stream_op_batch_finish_with_failure(
     grpc_exec_ctx *exec_ctx, grpc_transport_stream_op_batch *batch,
-    grpc_error *error) {
+    grpc_error *error, grpc_call_combiner *call_combiner) {
   if (batch->send_message) {
     grpc_byte_stream_destroy(exec_ctx,
                              batch->payload->send_message.send_message);
   }
   if (batch->recv_message) {
-    GRPC_CLOSURE_SCHED(exec_ctx,
-                       batch->payload->recv_message.recv_message_ready,
-                       GRPC_ERROR_REF(error));
+    GRPC_CALL_COMBINER_START(exec_ctx, call_combiner,
+                             batch->payload->recv_message.recv_message_ready,
+                             GRPC_ERROR_REF(error),
+                             "failing recv_message_ready");
   }
   if (batch->recv_initial_metadata) {
-    GRPC_CLOSURE_SCHED(
-        exec_ctx,
+    GRPC_CALL_COMBINER_START(
+        exec_ctx, call_combiner,
         batch->payload->recv_initial_metadata.recv_initial_metadata_ready,
-        GRPC_ERROR_REF(error));
+        GRPC_ERROR_REF(error), "failing recv_initial_metadata_ready");
   }
   GRPC_CLOSURE_SCHED(exec_ctx, batch->on_complete, error);
   if (batch->cancel_stream) {
diff --git a/src/core/lib/transport/transport.h b/src/core/lib/transport/transport.h
index 099138ea14..fbf5dcb8b5 100644
--- a/src/core/lib/transport/transport.h
+++ b/src/core/lib/transport/transport.h
@@ -22,6 +22,7 @@
 #include <stddef.h>
 
 #include "src/core/lib/channel/context.h"
+#include "src/core/lib/iomgr/call_combiner.h"
 #include "src/core/lib/iomgr/endpoint.h"
 #include "src/core/lib/iomgr/polling_entity.h"
 #include "src/core/lib/iomgr/pollset.h"
@@ -152,6 +153,9 @@ struct grpc_transport_stream_op_batch_payload {
     /** Iff send_initial_metadata != NULL, flags associated with
         send_initial_metadata: a bitfield of GRPC_INITIAL_METADATA_xxx */
     uint32_t send_initial_metadata_flags;
+    // If non-NULL, will be set by the transport to the peer string
+    // (a char*, which the caller takes ownership of).
+    gpr_atm *peer_string;
   } send_initial_metadata;
 
   struct {
@@ -176,6 +180,9 @@ struct grpc_transport_stream_op_batch_payload {
     // immediately available.  This may be a signal that we received a
     // Trailers-Only response.
     bool *trailing_metadata_available;
+    // If non-NULL, will be set by the transport to the peer string
+    // (a char*, which the caller takes ownership of).
+    gpr_atm *peer_string;
   } recv_initial_metadata;
 
   struct {
@@ -293,7 +300,7 @@ void grpc_transport_destroy_stream(grpc_exec_ctx *exec_ctx,
 
 void grpc_transport_stream_op_batch_finish_with_failure(
     grpc_exec_ctx *exec_ctx, grpc_transport_stream_op_batch *op,
-    grpc_error *error);
+    grpc_error *error, grpc_call_combiner *call_combiner);
 
 char *grpc_transport_stream_op_batch_string(grpc_transport_stream_op_batch *op);
 char *grpc_transport_op_string(grpc_transport_op *op);
@@ -332,10 +339,6 @@ void grpc_transport_close(grpc_transport *transport);
 /* Destroy the transport */
 void grpc_transport_destroy(grpc_exec_ctx *exec_ctx, grpc_transport *transport);
 
-/* Get the transports peer */
-char *grpc_transport_get_peer(grpc_exec_ctx *exec_ctx,
-                              grpc_transport *transport);
-
 /* Get the endpoint used by \a transport */
 grpc_endpoint *grpc_transport_get_endpoint(grpc_exec_ctx *exec_ctx,
                                            grpc_transport *transport);
diff --git a/src/core/lib/transport/transport_impl.h b/src/core/lib/transport/transport_impl.h
index fc772c6dd1..bbae69c223 100644
--- a/src/core/lib/transport/transport_impl.h
+++ b/src/core/lib/transport/transport_impl.h
@@ -59,9 +59,6 @@ typedef struct grpc_transport_vtable {
   /* implementation of grpc_transport_destroy */
   void (*destroy)(grpc_exec_ctx *exec_ctx, grpc_transport *self);
 
-  /* implementation of grpc_transport_get_peer */
-  char *(*get_peer)(grpc_exec_ctx *exec_ctx, grpc_transport *self);
-
   /* implementation of grpc_transport_get_endpoint */
   grpc_endpoint *(*get_endpoint)(grpc_exec_ctx *exec_ctx, grpc_transport *self);
 } grpc_transport_vtable;
diff --git a/src/core/lib/transport/transport_op_string.c b/src/core/lib/transport/transport_op_string.c
index 7b18229ba6..409a6c4103 100644
--- a/src/core/lib/transport/transport_op_string.c
+++ b/src/core/lib/transport/transport_op_string.c
@@ -112,6 +112,13 @@ char *grpc_transport_stream_op_batch_string(
     gpr_strvec_add(&b, tmp);
   }
 
+  if (op->collect_stats) {
+    gpr_strvec_add(&b, gpr_strdup(" "));
+    gpr_asprintf(&tmp, "COLLECT_STATS:%p",
+                 op->payload->collect_stats.collect_stats);
+    gpr_strvec_add(&b, tmp);
+  }
+
   out = gpr_strvec_flatten(&b, NULL);
   gpr_strvec_destroy(&b);
 
diff --git a/src/cpp/common/channel_filter.cc b/src/cpp/common/channel_filter.cc
index 448d9fbcf2..ea44cff832 100644
--- a/src/cpp/common/channel_filter.cc
+++ b/src/cpp/common/channel_filter.cc
@@ -68,10 +68,6 @@ void CallData::SetPollsetOrPollsetSet(grpc_exec_ctx *exec_ctx,
   grpc_call_stack_ignore_set_pollset_or_pollset_set(exec_ctx, elem, pollent);
 }
 
-char *CallData::GetPeer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  return grpc_call_next_get_peer(exec_ctx, elem);
-}
-
 // internal code used by RegisterChannelFilter()
 
 namespace internal {
diff --git a/src/cpp/common/channel_filter.h b/src/cpp/common/channel_filter.h
index c3d187d7e1..c1aeb3f724 100644
--- a/src/cpp/common/channel_filter.h
+++ b/src/cpp/common/channel_filter.h
@@ -268,9 +268,6 @@ class CallData {
   virtual void SetPollsetOrPollsetSet(grpc_exec_ctx *exec_ctx,
                                       grpc_call_element *elem,
                                       grpc_polling_entity *pollent);
-
-  /// Gets the peer name.
-  virtual char *GetPeer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem);
 };
 
 namespace internal {
@@ -349,11 +346,6 @@ class ChannelFilter final {
     CallDataType *call_data = reinterpret_cast<CallDataType *>(elem->call_data);
     call_data->SetPollsetOrPollsetSet(exec_ctx, elem, pollent);
   }
-
-  static char *GetPeer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-    CallDataType *call_data = reinterpret_cast<CallDataType *>(elem->call_data);
-    return call_data->GetPeer(exec_ctx, elem);
-  }
 };
 
 struct FilterRecord {
@@ -396,8 +388,7 @@ void RegisterChannelFilter(
        FilterType::call_data_size, FilterType::InitCallElement,
        FilterType::SetPollsetOrPollsetSet, FilterType::DestroyCallElement,
        FilterType::channel_data_size, FilterType::InitChannelElement,
-       FilterType::DestroyChannelElement, FilterType::GetPeer,
-       FilterType::GetChannelInfo, name}};
+       FilterType::DestroyChannelElement, FilterType::GetChannelInfo, name}};
   internal::channel_filters->push_back(filter_record);
 }
 
diff --git a/src/python/grpcio/grpc_core_dependencies.py b/src/python/grpcio/grpc_core_dependencies.py
index 1cbf345ab6..39acd5ddbc 100644
--- a/src/python/grpcio/grpc_core_dependencies.py
+++ b/src/python/grpcio/grpc_core_dependencies.py
@@ -77,6 +77,7 @@ CORE_SOURCE_FILES = [
   'src/core/lib/http/format_request.c',
   'src/core/lib/http/httpcli.c',
   'src/core/lib/http/parser.c',
+  'src/core/lib/iomgr/call_combiner.c',
   'src/core/lib/iomgr/closure.c',
   'src/core/lib/iomgr/combiner.c',
   'src/core/lib/iomgr/endpoint.c',
diff --git a/test/core/channel/channel_stack_test.c b/test/core/channel/channel_stack_test.c
index 0c4bae6ded..7c3614b4a2 100644
--- a/test/core/channel/channel_stack_test.c
+++ b/test/core/channel/channel_stack_test.c
@@ -67,10 +67,6 @@ static void channel_func(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
   ++*(int *)(elem->channel_data);
 }
 
-static char *get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  return gpr_strdup("peer");
-}
-
 static void free_channel(grpc_exec_ctx *exec_ctx, void *arg,
                          grpc_error *error) {
   grpc_channel_stack_destroy(exec_ctx, arg);
@@ -93,7 +89,6 @@ static void test_create_channel_stack(void) {
       sizeof(int),
       channel_init_func,
       channel_destroy_func,
-      get_peer,
       grpc_channel_next_get_info,
       "some_test_filter"};
   const grpc_channel_filter *filters = &filter;
diff --git a/test/core/channel/minimal_stack_is_minimal_test.c b/test/core/channel/minimal_stack_is_minimal_test.c
index c99b54c6ac..b4528346f7 100644
--- a/test/core/channel/minimal_stack_is_minimal_test.c
+++ b/test/core/channel/minimal_stack_is_minimal_test.c
@@ -89,14 +89,14 @@ int main(int argc, char **argv) {
                         "connected", NULL);
   errors += CHECK_STACK("unknown", NULL, GRPC_SERVER_CHANNEL, "server",
                         "message_size", "deadline", "connected", NULL);
-  errors +=
-      CHECK_STACK("chttp2", NULL, GRPC_CLIENT_DIRECT_CHANNEL, "message_size",
-                  "deadline", "http-client", "compress", "connected", NULL);
+  errors += CHECK_STACK("chttp2", NULL, GRPC_CLIENT_DIRECT_CHANNEL,
+                        "message_size", "deadline", "http-client",
+                        "message_compress", "connected", NULL);
   errors += CHECK_STACK("chttp2", NULL, GRPC_CLIENT_SUBCHANNEL, "message_size",
-                        "http-client", "compress", "connected", NULL);
-  errors +=
-      CHECK_STACK("chttp2", NULL, GRPC_SERVER_CHANNEL, "server", "message_size",
-                  "deadline", "http-server", "compress", "connected", NULL);
+                        "http-client", "message_compress", "connected", NULL);
+  errors += CHECK_STACK("chttp2", NULL, GRPC_SERVER_CHANNEL, "server",
+                        "message_size", "deadline", "http-server",
+                        "message_compress", "connected", NULL);
   errors +=
       CHECK_STACK(NULL, NULL, GRPC_CLIENT_CHANNEL, "client-channel", NULL);
 
diff --git a/test/core/end2end/tests/filter_call_init_fails.c b/test/core/end2end/tests/filter_call_init_fails.c
index b6be375a51..09e9dbcd7b 100644
--- a/test/core/end2end/tests/filter_call_init_fails.c
+++ b/test/core/end2end/tests/filter_call_init_fails.c
@@ -430,7 +430,6 @@ static const grpc_channel_filter test_filter = {
     0,
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "filter_call_init_fails"};
 
diff --git a/test/core/end2end/tests/filter_causes_close.c b/test/core/end2end/tests/filter_causes_close.c
index aff39dd89d..5a8c96d121 100644
--- a/test/core/end2end/tests/filter_causes_close.c
+++ b/test/core/end2end/tests/filter_causes_close.c
@@ -197,7 +197,7 @@ static void recv_im_ready(grpc_exec_ctx *exec_ctx, void *arg,
                           grpc_error *error) {
   grpc_call_element *elem = arg;
   call_data *calld = elem->call_data;
-  GRPC_CLOSURE_SCHED(
+  GRPC_CLOSURE_RUN(
       exec_ctx, calld->recv_im_ready,
       grpc_error_set_int(GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
                              "Failure that's not preventable.", &error, 1),
@@ -247,7 +247,6 @@ static const grpc_channel_filter test_filter = {
     sizeof(channel_data),
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "filter_causes_close"};
 
diff --git a/test/core/end2end/tests/filter_latency.c b/test/core/end2end/tests/filter_latency.c
index 5dbbc4d18d..8918c3b2f6 100644
--- a/test/core/end2end/tests/filter_latency.c
+++ b/test/core/end2end/tests/filter_latency.c
@@ -290,7 +290,6 @@ static const grpc_channel_filter test_client_filter = {
     0,
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "client_filter_latency"};
 
@@ -304,7 +303,6 @@ static const grpc_channel_filter test_server_filter = {
     0,
     init_channel_elem,
     destroy_channel_elem,
-    grpc_call_next_get_peer,
     grpc_channel_next_get_info,
     "server_filter_latency"};
 
diff --git a/test/cpp/microbenchmarks/bm_call_create.cc b/test/cpp/microbenchmarks/bm_call_create.cc
index 508f7f94d6..518c65ac8d 100644
--- a/test/cpp/microbenchmarks/bm_call_create.cc
+++ b/test/cpp/microbenchmarks/bm_call_create.cc
@@ -39,6 +39,7 @@ extern "C" {
 #include "src/core/ext/filters/message_size/message_size_filter.h"
 #include "src/core/lib/channel/channel_stack.h"
 #include "src/core/lib/channel/connected_channel.h"
+#include "src/core/lib/iomgr/call_combiner.h"
 #include "src/core/lib/profiling/timers.h"
 #include "src/core/lib/surface/channel.h"
 #include "src/core/lib/transport/transport_impl.h"
@@ -396,10 +397,6 @@ grpc_error *InitChannelElem(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
 
 void DestroyChannelElem(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem) {}
 
-char *GetPeer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  return gpr_strdup("peer");
-}
-
 void GetChannelInfo(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
                     const grpc_channel_info *channel_info) {}
 
@@ -412,7 +409,6 @@ static const grpc_channel_filter dummy_filter = {StartTransportStreamOp,
                                                  0,
                                                  InitChannelElem,
                                                  DestroyChannelElem,
-                                                 GetPeer,
                                                  GetChannelInfo,
                                                  "dummy_filter"};
 
@@ -459,11 +455,6 @@ void DestroyStream(grpc_exec_ctx *exec_ctx, grpc_transport *self,
 /* implementation of grpc_transport_destroy */
 void Destroy(grpc_exec_ctx *exec_ctx, grpc_transport *self) {}
 
-/* implementation of grpc_transport_get_peer */
-char *GetPeer(grpc_exec_ctx *exec_ctx, grpc_transport *self) {
-  return gpr_strdup("transport_peer");
-}
-
 /* implementation of grpc_transport_get_endpoint */
 grpc_endpoint *GetEndpoint(grpc_exec_ctx *exec_ctx, grpc_transport *self) {
   return nullptr;
@@ -473,7 +464,7 @@ static const grpc_transport_vtable dummy_transport_vtable = {
     0,          "dummy_http2", InitStream,
     SetPollset, SetPollsetSet, PerformStreamOp,
     PerformOp,  DestroyStream, Destroy,
-    GetPeer,    GetEndpoint};
+    GetEndpoint};
 
 static grpc_transport dummy_transport = {&dummy_transport_vtable};
 
@@ -639,18 +630,22 @@ BENCHMARK_TEMPLATE(BM_IsolatedFilter, LoadReportingFilter, SendEmptyMetadata);
 
 namespace isolated_call_filter {
 
+typedef struct { grpc_call_combiner *call_combiner; } call_data;
+
 static void StartTransportStreamOp(grpc_exec_ctx *exec_ctx,
                                    grpc_call_element *elem,
                                    grpc_transport_stream_op_batch *op) {
+  call_data *calld = static_cast<call_data *>(elem->call_data);
   if (op->recv_initial_metadata) {
-    GRPC_CLOSURE_SCHED(
-        exec_ctx,
+    GRPC_CALL_COMBINER_START(
+        exec_ctx, calld->call_combiner,
         op->payload->recv_initial_metadata.recv_initial_metadata_ready,
-        GRPC_ERROR_NONE);
+        GRPC_ERROR_NONE, "recv_initial_metadata");
   }
   if (op->recv_message) {
-    GRPC_CLOSURE_SCHED(exec_ctx, op->payload->recv_message.recv_message_ready,
-                       GRPC_ERROR_NONE);
+    GRPC_CALL_COMBINER_START(exec_ctx, calld->call_combiner,
+                             op->payload->recv_message.recv_message_ready,
+                             GRPC_ERROR_NONE, "recv_message");
   }
   GRPC_CLOSURE_SCHED(exec_ctx, op->on_complete, GRPC_ERROR_NONE);
 }
@@ -667,6 +662,8 @@ static void StartTransportOp(grpc_exec_ctx *exec_ctx,
 static grpc_error *InitCallElem(grpc_exec_ctx *exec_ctx,
                                 grpc_call_element *elem,
                                 const grpc_call_element_args *args) {
+  call_data *calld = static_cast<call_data *>(elem->call_data);
+  calld->call_combiner = args->call_combiner;
   return GRPC_ERROR_NONE;
 }
 
@@ -687,24 +684,19 @@ grpc_error *InitChannelElem(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
 
 void DestroyChannelElem(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem) {}
 
-char *GetPeer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  return gpr_strdup("peer");
-}
-
 void GetChannelInfo(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
                     const grpc_channel_info *channel_info) {}
 
 static const grpc_channel_filter isolated_call_filter = {
     StartTransportStreamOp,
     StartTransportOp,
-    0,
+    sizeof(call_data),
     InitCallElem,
     SetPollsetOrPollsetSet,
     DestroyCallElem,
     0,
     InitChannelElem,
     DestroyChannelElem,
-    GetPeer,
     GetChannelInfo,
     "isolated_call_filter"};
 }  // namespace isolated_call_filter
diff --git a/test/cpp/microbenchmarks/bm_chttp2_transport.cc b/test/cpp/microbenchmarks/bm_chttp2_transport.cc
index cb113c5254..936681fec1 100644
--- a/test/cpp/microbenchmarks/bm_chttp2_transport.cc
+++ b/test/cpp/microbenchmarks/bm_chttp2_transport.cc
@@ -286,6 +286,7 @@ static void BM_StreamCreateSendInitialMetadataDestroy(benchmark::State &state) {
   Stream s(&f);
   grpc_transport_stream_op_batch op;
   grpc_transport_stream_op_batch_payload op_payload;
+  memset(&op_payload, 0, sizeof(op_payload));
   std::unique_ptr<Closure> start;
   std::unique_ptr<Closure> done;
 
@@ -337,6 +338,7 @@ static void BM_TransportEmptyOp(benchmark::State &state) {
   s.Init(state);
   grpc_transport_stream_op_batch op;
   grpc_transport_stream_op_batch_payload op_payload;
+  memset(&op_payload, 0, sizeof(op_payload));
   auto reset_op = [&]() {
     memset(&op, 0, sizeof(op));
     op.payload = &op_payload;
@@ -364,6 +366,7 @@ static void BM_TransportStreamSend(benchmark::State &state) {
   s.Init(state);
   grpc_transport_stream_op_batch op;
   grpc_transport_stream_op_batch_payload op_payload;
+  memset(&op_payload, 0, sizeof(op_payload));
   auto reset_op = [&]() {
     memset(&op, 0, sizeof(op));
     op.payload = &op_payload;
@@ -485,6 +488,7 @@ static void BM_TransportStreamRecv(benchmark::State &state) {
   Stream s(&f);
   s.Init(state);
   grpc_transport_stream_op_batch_payload op_payload;
+  memset(&op_payload, 0, sizeof(op_payload));
   grpc_transport_stream_op_batch op;
   grpc_byte_stream *recv_stream;
   grpc_slice incoming_data = CreateIncomingDataSlice(state.range(0), 16384);
diff --git a/tools/doxygen/Doxyfile.c++.internal b/tools/doxygen/Doxyfile.c++.internal
index 91c149eec9..c2e369afe5 100644
--- a/tools/doxygen/Doxyfile.c++.internal
+++ b/tools/doxygen/Doxyfile.c++.internal
@@ -948,6 +948,7 @@ src/core/lib/debug/trace.h \
 src/core/lib/http/format_request.h \
 src/core/lib/http/httpcli.h \
 src/core/lib/http/parser.h \
+src/core/lib/iomgr/call_combiner.h \
 src/core/lib/iomgr/closure.h \
 src/core/lib/iomgr/combiner.h \
 src/core/lib/iomgr/endpoint.h \
diff --git a/tools/doxygen/Doxyfile.core.internal b/tools/doxygen/Doxyfile.core.internal
index 26d982acd7..94bbe3185a 100644
--- a/tools/doxygen/Doxyfile.core.internal
+++ b/tools/doxygen/Doxyfile.core.internal
@@ -1091,6 +1091,8 @@ src/core/lib/http/httpcli_security_connector.c \
 src/core/lib/http/parser.c \
 src/core/lib/http/parser.h \
 src/core/lib/iomgr/README.md \
+src/core/lib/iomgr/call_combiner.c \
+src/core/lib/iomgr/call_combiner.h \
 src/core/lib/iomgr/closure.c \
 src/core/lib/iomgr/closure.h \
 src/core/lib/iomgr/combiner.c \
diff --git a/tools/run_tests/generated/sources_and_headers.json b/tools/run_tests/generated/sources_and_headers.json
index 5ce25dc14a..7b8cc27a60 100644
--- a/tools/run_tests/generated/sources_and_headers.json
+++ b/tools/run_tests/generated/sources_and_headers.json
@@ -7827,6 +7827,7 @@
       "src/core/lib/http/format_request.c", 
       "src/core/lib/http/httpcli.c", 
       "src/core/lib/http/parser.c", 
+      "src/core/lib/iomgr/call_combiner.c", 
       "src/core/lib/iomgr/closure.c", 
       "src/core/lib/iomgr/combiner.c", 
       "src/core/lib/iomgr/endpoint.c", 
@@ -7980,6 +7981,7 @@
       "src/core/lib/http/format_request.h", 
       "src/core/lib/http/httpcli.h", 
       "src/core/lib/http/parser.h", 
+      "src/core/lib/iomgr/call_combiner.h", 
       "src/core/lib/iomgr/closure.h", 
       "src/core/lib/iomgr/combiner.h", 
       "src/core/lib/iomgr/endpoint.h", 
@@ -8112,6 +8114,7 @@
       "src/core/lib/http/format_request.h", 
       "src/core/lib/http/httpcli.h", 
       "src/core/lib/http/parser.h", 
+      "src/core/lib/iomgr/call_combiner.h", 
       "src/core/lib/iomgr/closure.h", 
       "src/core/lib/iomgr/combiner.h", 
       "src/core/lib/iomgr/endpoint.h",