Merge github.com:grpc/grpc into write_completion

37 files changed, 1307 insertions, 864 deletions

diff --git a/src/core/ext/census/context.c b/src/core/ext/census/context.c
index 1019b287d7..9b25a32e36 100644
--- a/src/core/ext/census/context.c
+++ b/src/core/ext/census/context.c
@@ -141,7 +141,7 @@ static char *decode_tag(struct raw_tag *tag, char *header, int offset) {
 // Make a copy (in 'to') of an existing tag_set.
 static void tag_set_copy(struct tag_set *to, const struct tag_set *from) {
   memcpy(to, from, sizeof(struct tag_set));
-  to->kvm = gpr_malloc(to->kvm_size);
+  to->kvm = (char *)gpr_malloc(to->kvm_size);
   memcpy(to->kvm, from->kvm, from->kvm_used);
 }
 
@@ -184,7 +184,7 @@ static bool tag_set_add_tag(struct tag_set *tags, const census_tag *tag,
   if (tags->kvm_used + tag_size > tags->kvm_size) {
     // allocate new memory if needed
     tags->kvm_size += 2 * CENSUS_MAX_TAG_KV_LEN + TAG_HEADER_SIZE;
-    char *new_kvm = gpr_malloc(tags->kvm_size);
+    char *new_kvm = (char *)gpr_malloc(tags->kvm_size);
     if (tags->kvm_used > 0) memcpy(new_kvm, tags->kvm, tags->kvm_used);
     gpr_free(tags->kvm);
     tags->kvm = new_kvm;
@@ -274,7 +274,8 @@ static void tag_set_flatten(struct tag_set *tags) {
 census_context *census_context_create(const census_context *base,
                                       const census_tag *tags, int ntags,
                                       census_context_status const **status) {
-  census_context *context = gpr_malloc(sizeof(census_context));
+  census_context *context =
+      (census_context *)gpr_malloc(sizeof(census_context));
   // If we are given a base, copy it into our new tag set. Otherwise set it
   // to zero/NULL everything.
   if (base == NULL) {
@@ -459,7 +460,7 @@ static void tag_set_decode(struct tag_set *tags, const char *buffer,
   }
   tags->kvm_used = size - header_size;
   tags->kvm_size = tags->kvm_used + CENSUS_MAX_TAG_KV_LEN;
-  tags->kvm = gpr_malloc(tags->kvm_size);
+  tags->kvm = (char *)gpr_malloc(tags->kvm_size);
   if (tag_header_size != TAG_HEADER_SIZE) {
     // something new in the tag information. I don't understand it, so
     // don't copy it over.
@@ -481,7 +482,8 @@ static void tag_set_decode(struct tag_set *tags, const char *buffer,
 }
 
 census_context *census_context_decode(const char *buffer, size_t size) {
-  census_context *context = gpr_malloc(sizeof(census_context));
+  census_context *context =
+      (census_context *)gpr_malloc(sizeof(census_context));
   memset(&context->tags[LOCAL_TAGS], 0, sizeof(struct tag_set));
   if (buffer == NULL) {
     memset(&context->tags[PROPAGATED_TAGS], 0, sizeof(struct tag_set));
diff --git a/src/core/ext/census/grpc_filter.c b/src/core/ext/census/grpc_filter.c
index 13fe2e6b1c..b37ab90389 100644
--- a/src/core/ext/census/grpc_filter.c
+++ b/src/core/ext/census/grpc_filter.c
@@ -60,8 +60,8 @@ static void extract_and_annotate_method_tag(grpc_metadata_batch *md,
 
 static void client_mutate_op(grpc_call_element *elem,
                              grpc_transport_stream_op_batch *op) {
-  call_data *calld = elem->call_data;
-  channel_data *chand = elem->channel_data;
+  call_data *calld = (call_data *)elem->call_data;
+  channel_data *chand = (channel_data *)elem->channel_data;
   if (op->send_initial_metadata) {
     extract_and_annotate_method_tag(
         op->payload->send_initial_metadata.send_initial_metadata, calld, chand);
@@ -78,9 +78,9 @@ static void client_start_transport_op(grpc_exec_ctx *exec_ctx,
 static void server_on_done_recv(grpc_exec_ctx *exec_ctx, void *ptr,
                                 grpc_error *error) {
   GPR_TIMER_BEGIN("census-server:server_on_done_recv", 0);
-  grpc_call_element *elem = ptr;
-  call_data *calld = elem->call_data;
-  channel_data *chand = elem->channel_data;
+  grpc_call_element *elem = (grpc_call_element *)ptr;
+  call_data *calld = (call_data *)elem->call_data;
+  channel_data *chand = (channel_data *)elem->channel_data;
   if (error == GRPC_ERROR_NONE) {
     extract_and_annotate_method_tag(calld->recv_initial_metadata, calld, chand);
   }
@@ -90,7 +90,7 @@ static void server_on_done_recv(grpc_exec_ctx *exec_ctx, void *ptr,
 
 static void server_mutate_op(grpc_call_element *elem,
                              grpc_transport_stream_op_batch *op) {
-  call_data *calld = elem->call_data;
+  call_data *calld = (call_data *)elem->call_data;
   if (op->recv_initial_metadata) {
     /* substitute our callback for the op callback */
     calld->recv_initial_metadata =
@@ -117,7 +117,7 @@ static void server_start_transport_op(grpc_exec_ctx *exec_ctx,
 static grpc_error *client_init_call_elem(grpc_exec_ctx *exec_ctx,
                                          grpc_call_element *elem,
                                          const grpc_call_element_args *args) {
-  call_data *d = elem->call_data;
+  call_data *d = (call_data *)elem->call_data;
   GPR_ASSERT(d != NULL);
   memset(d, 0, sizeof(*d));
   d->start_ts = args->start_time;
@@ -128,7 +128,7 @@ static void client_destroy_call_elem(grpc_exec_ctx *exec_ctx,
                                      grpc_call_element *elem,
                                      const grpc_call_final_info *final_info,
                                      grpc_closure *ignored) {
-  call_data *d = elem->call_data;
+  call_data *d = (call_data *)elem->call_data;
   GPR_ASSERT(d != NULL);
   /* TODO(hongyu): record rpc client stats and census_rpc_end_op here */
 }
@@ -136,7 +136,7 @@ static void client_destroy_call_elem(grpc_exec_ctx *exec_ctx,
 static grpc_error *server_init_call_elem(grpc_exec_ctx *exec_ctx,
                                          grpc_call_element *elem,
                                          const grpc_call_element_args *args) {
-  call_data *d = elem->call_data;
+  call_data *d = (call_data *)elem->call_data;
   GPR_ASSERT(d != NULL);
   memset(d, 0, sizeof(*d));
   d->start_ts = args->start_time;
@@ -150,7 +150,7 @@ static void server_destroy_call_elem(grpc_exec_ctx *exec_ctx,
                                      grpc_call_element *elem,
                                      const grpc_call_final_info *final_info,
                                      grpc_closure *ignored) {
-  call_data *d = elem->call_data;
+  call_data *d = (call_data *)elem->call_data;
   GPR_ASSERT(d != NULL);
   /* TODO(hongyu): record rpc server stats and census_tracing_end_op here */
 }
@@ -158,14 +158,14 @@ static void server_destroy_call_elem(grpc_exec_ctx *exec_ctx,
 static grpc_error *init_channel_elem(grpc_exec_ctx *exec_ctx,
                                      grpc_channel_element *elem,
                                      grpc_channel_element_args *args) {
-  channel_data *chand = elem->channel_data;
+  channel_data *chand = (channel_data *)elem->channel_data;
   GPR_ASSERT(chand != NULL);
   return GRPC_ERROR_NONE;
 }
 
 static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
                                  grpc_channel_element *elem) {
-  channel_data *chand = elem->channel_data;
+  channel_data *chand = (channel_data *)elem->channel_data;
   GPR_ASSERT(chand != NULL);
 }
 
@@ -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/census/mlog.c b/src/core/ext/census/mlog.c
index 937ceb101b..4b8c8466b3 100644
--- a/src/core/ext/census/mlog.c
+++ b/src/core/ext/census/mlog.c
@@ -467,7 +467,8 @@ void census_log_initialize(size_t size_in_mb, int discard_old_records) {
   g_log.blocks = (cl_block*)gpr_malloc_aligned(
       g_log.num_blocks * sizeof(cl_block), GPR_CACHELINE_SIZE_LOG);
   memset(g_log.blocks, 0, g_log.num_blocks * sizeof(cl_block));
-  g_log.buffer = gpr_malloc(g_log.num_blocks * CENSUS_LOG_MAX_RECORD_SIZE);
+  g_log.buffer =
+      (char*)gpr_malloc(g_log.num_blocks * CENSUS_LOG_MAX_RECORD_SIZE);
   memset(g_log.buffer, 0, g_log.num_blocks * CENSUS_LOG_MAX_RECORD_SIZE);
   cl_block_list_initialize(&g_log.free_block_list);
   cl_block_list_initialize(&g_log.dirty_block_list);
diff --git a/src/core/ext/census/resource.c b/src/core/ext/census/resource.c
index 1a676f0e1e..44a887231c 100644
--- a/src/core/ext/census/resource.c
+++ b/src/core/ext/census/resource.c
@@ -87,7 +87,7 @@ static bool validate_string(pb_istream_t *stream, const pb_field_t *field,
         gpr_log(GPR_INFO, "Zero-length Resource name.");
         return false;
       }
-      vresource->name = gpr_malloc(stream->bytes_left + 1);
+      vresource->name = (char *)gpr_malloc(stream->bytes_left + 1);
       vresource->name[stream->bytes_left] = '\0';
       if (!pb_read(stream, (uint8_t *)vresource->name, stream->bytes_left)) {
         return false;
@@ -106,7 +106,7 @@ static bool validate_string(pb_istream_t *stream, const pb_field_t *field,
       if (stream->bytes_left == 0) {
         return true;
       }
-      vresource->description = gpr_malloc(stream->bytes_left + 1);
+      vresource->description = (char *)gpr_malloc(stream->bytes_left + 1);
       vresource->description[stream->bytes_left] = '\0';
       if (!pb_read(stream, (uint8_t *)vresource->description,
                    stream->bytes_left)) {
@@ -134,7 +134,8 @@ static bool validate_units_helper(pb_istream_t *stream, int *count,
     // Have to allocate a new array of values. Normal case is 0 or 1, so
     // this should normally not be an issue.
     google_census_Resource_BasicUnit *new_bup =
-        gpr_malloc((size_t)*count * sizeof(google_census_Resource_BasicUnit));
+        (google_census_Resource_BasicUnit *)gpr_malloc(
+            (size_t)*count * sizeof(google_census_Resource_BasicUnit));
     if (*count != 1) {
       memcpy(new_bup, *bup,
              (size_t)(*count - 1) * sizeof(google_census_Resource_BasicUnit));
@@ -207,7 +208,8 @@ size_t allocate_resource(void) {
   // Expand resources if needed.
   if (n_resources == n_defined_resources) {
     size_t new_n_resources = n_resources ? n_resources * 2 : 2;
-    resource **new_resources = gpr_malloc(new_n_resources * sizeof(resource *));
+    resource **new_resources =
+        (resource **)gpr_malloc(new_n_resources * sizeof(resource *));
     if (n_resources != 0) {
       memcpy(new_resources, resources, n_resources * sizeof(resource *));
     }
@@ -226,7 +228,7 @@ size_t allocate_resource(void) {
     }
   }
   GPR_ASSERT(id < n_resources && resources[id] == NULL);
-  resources[id] = gpr_malloc(sizeof(resource));
+  resources[id] = (resource *)gpr_malloc(sizeof(resource));
   memset(resources[id], 0, sizeof(resource));
   n_defined_resources++;
   next_id = (id + 1) % n_resources;
@@ -276,22 +278,24 @@ int32_t define_resource(const resource *base) {
   gpr_mu_lock(&resource_lock);
   size_t id = allocate_resource();
   size_t len = strlen(base->name) + 1;
-  resources[id]->name = gpr_malloc(len);
+  resources[id]->name = (char *)gpr_malloc(len);
   memcpy(resources[id]->name, base->name, len);
   if (base->description) {
     len = strlen(base->description) + 1;
-    resources[id]->description = gpr_malloc(len);
+    resources[id]->description = (char *)gpr_malloc(len);
     memcpy(resources[id]->description, base->description, len);
   }
   resources[id]->prefix = base->prefix;
   resources[id]->n_numerators = base->n_numerators;
   len = (size_t)base->n_numerators * sizeof(*base->numerators);
-  resources[id]->numerators = gpr_malloc(len);
+  resources[id]->numerators =
+      (google_census_Resource_BasicUnit *)gpr_malloc(len);
   memcpy(resources[id]->numerators, base->numerators, len);
   resources[id]->n_denominators = base->n_denominators;
   if (base->n_denominators != 0) {
     len = (size_t)base->n_denominators * sizeof(*base->denominators);
-    resources[id]->denominators = gpr_malloc(len);
+    resources[id]->denominators =
+        (google_census_Resource_BasicUnit *)gpr_malloc(len);
     memcpy(resources[id]->denominators, base->denominators, len);
   }
   gpr_mu_unlock(&resource_lock);
diff --git a/src/core/ext/filters/client_channel/client_channel.c b/src/core/ext/filters/client_channel/client_channel.c
index 58e31d7b45..c24e52ec1d 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.
@@ -807,24 +808,27 @@ typedef struct client_channel_call_data {
   // State for handling deadlines.
   // 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.
+  // and this struct both independently store pointers to the call stack
+  // and call 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_stack *owning_call;
+  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 lb_pick_cancel_closure;
 
   grpc_connected_subchannel *connected_subchannel;
   grpc_call_context_element subchannel_call_context[GRPC_CONTEXT_COUNT];
@@ -832,10 +836,9 @@ 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 +846,42 @@ 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 +890,60 @@ 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 +984,28 @@ 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 +1017,27 @@ 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);
-    }
-    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));
+              calld, grpc_error_string(calld->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
@@ -1065,39 +1047,78 @@ static bool pick_subchannel_locked(grpc_exec_ctx *exec_ctx,
 
 typedef struct {
   grpc_call_element *elem;
-  bool cancelled;
+  bool finished;
   grpc_closure closure;
+  grpc_closure cancel_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) {
+  pick_after_resolver_result_args *args = arg;
+  if (args->finished) {
+    gpr_free(args);
+    return;
+  }
+  args->finished = true;
+  grpc_call_element *elem = args->elem;
+  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
+  // finished, 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.
+  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
+    gpr_log(GPR_DEBUG,
+            "chand=%p calld=%p: cancelling pick waiting for resolver result",
+            chand, calld);
+  }
+  // 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) {
   pick_after_resolver_result_args *args = arg;
-  if (args->cancelled) {
+  if (args->finished) {
     /* cancelled, do nothing */
     if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
       gpr_log(GPR_DEBUG, "call cancelled before resolver result");
     }
+    gpr_free(args);
+    return;
+  }
+  args->finished = true;
+  grpc_call_element *elem = args->elem;
+  channel_data *chand = elem->channel_data;
+  call_data *calld = elem->call_data;
+  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, elem, GRPC_ERROR_REF(error));
   } else {
-    channel_data *chand = args->elem->channel_data;
-    call_data *calld = args->elem->call_data;
-    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));
-    } 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 (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, elem)) {
+      subchannel_ready_locked(exec_ctx, elem, GRPC_ERROR_NONE);
     }
   }
-  gpr_free(args);
 }
 
 static void pick_after_resolver_result_start_locked(grpc_exec_ctx *exec_ctx,
@@ -1116,41 +1137,34 @@ 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(&args->cancel_closure,
+                        pick_after_resolver_result_cancel_locked, args,
+                        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 (error != GRPC_ERROR_NONE && 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);
+  GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call, "pick_callback_cancel");
 }
 
 // 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;
@@ -1194,24 +1208,17 @@ 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_STACK_REF(calld->owning_call, "pick_callback_cancel");
+    grpc_call_combiner_set_notify_on_cancel(
+        exec_ctx, calld->call_combiner,
+        GRPC_CLOSURE_INIT(&calld->lb_pick_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 +1231,7 @@ 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 +1248,7 @@ 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 +1265,33 @@ 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 +1314,49 @@ 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 +1366,43 @@ 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 +1417,12 @@ 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->owning_call = args->call_stack;
+  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 +1441,12 @@ 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 +1485,6 @@ 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..6b5b383efd 100644
--- a/src/core/ext/filters/client_channel/subchannel.c
+++ b/src/core/ext/filters/client_channel/subchannel.c
@@ -724,20 +724,14 @@ 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) {
+                                     grpc_transport_stream_op_batch *batch) {
   GPR_TIMER_BEGIN("grpc_subchannel_call_process_op", 0);
   grpc_call_stack *call_stack = SUBCHANNEL_CALL_TO_CALL_STACK(call);
   grpc_call_element *top_elem = grpc_call_stack_element(call_stack, 0);
-  top_elem->filter->start_transport_stream_op_batch(exec_ctx, top_elem, op);
+  GRPC_CALL_LOG_OP(GPR_INFO, top_elem, batch);
+  top_elem->filter->start_transport_stream_op_batch(exec_ctx, top_elem, batch);
   GPR_TIMER_END("grpc_subchannel_call_process_op", 0);
 }
 
@@ -760,13 +754,15 @@ 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..2d7429c41e 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
@@ -302,7 +303,6 @@ static void hc_start_transport_stream_op_batch(
   call_data *calld = elem->call_data;
   channel_data *channeld = elem->channel_data;
   GPR_TIMER_BEGIN("hc_start_transport_stream_op_batch", 0);
-  GRPC_CALL_LOG_OP(GPR_INFO, elem, batch);
 
   if (batch->recv_initial_metadata) {
     /* substitute our callback for the higher callback */
@@ -414,7 +414,7 @@ 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 +426,7 @@ 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 +566,5 @@ 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..450f67746f 100644
--- a/src/core/ext/filters/max_age/max_age_filter.c
+++ b/src/core/ext/filters/max_age/max_age_filter.c
@@ -273,7 +273,7 @@ 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) {
-  channel_data* chand = elem->channel_data;
+  channel_data* chand = (channel_data*)elem->channel_data;
   decrease_call_count(exec_ctx, chand);
 }
 
@@ -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 9d9e4ea2ba..8d1d13220c 100644
--- a/src/core/ext/transport/chttp2/transport/chttp2_transport.c
+++ b/src/core/ext/transport/chttp2/transport/chttp2_transport.c
@@ -1408,17 +1408,28 @@ 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);
@@ -1503,6 +1514,10 @@ 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);
   }
 
@@ -1862,8 +1877,7 @@ 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);
@@ -2975,14 +2989,6 @@ 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,
@@ -2999,7 +3005,6 @@ static const grpc_transport_vtable vtable = {sizeof(grpc_chttp2_stream),
                                              perform_transport_op,
                                              destroy_stream,
                                              destroy_transport,
-                                             chttp2_get_peer,
                                              chttp2_get_endpoint};
 
 static const grpc_transport_vtable *get_vtable(void) { return &vtable; }
diff --git a/src/core/ext/transport/chttp2/transport/internal.h b/src/core/ext/transport/chttp2/transport/internal.h
index f0962a6b0e..0fbedd1e56 100644
--- a/src/core/ext/transport/chttp2/transport/internal.h
+++ b/src/core/ext/transport/chttp2/transport/internal.h
@@ -514,6 +514,8 @@ 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 608c94aa30..6c12c91365 100644
--- a/src/core/ext/transport/chttp2/transport/parsing.c
+++ b/src/core/ext/transport/chttp2/transport/parsing.c
@@ -625,6 +625,7 @@ 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;
@@ -633,6 +634,7 @@ 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 765c13c65e..587a3b83b5 100644
--- a/src/core/ext/transport/cronet/transport/cronet_transport.c
+++ b/src/core/ext/transport/cronet/transport/cronet_transport.c
@@ -1429,10 +1429,6 @@ 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;
@@ -1451,7 +1447,6 @@ 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..48d8eaec18
--- /dev/null
+++ b/src/core/lib/iomgr/call_combiner.c
@@ -0,0 +1,202 @@
+/*
+ *
+ * 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) {
+      if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+        gpr_log(GPR_DEBUG,
+                "call_combiner=%p: scheduling notify_on_cancel callback=%p "
+                "for pre-existing cancellation",
+                call_combiner, closure);
+      }
+      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)) {
+        if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+          gpr_log(GPR_DEBUG, "call_combiner=%p: setting notify_on_cancel=%p",
+                  call_combiner, closure);
+        }
+        // If we replaced an earlier closure, invoke the original
+        // closure with GRPC_ERROR_NONE.  This allows callers to clean
+        // up any resources they may be holding for the callback.
+        if (original_state != 0) {
+          closure = (grpc_closure*)original_state;
+          if (GRPC_TRACER_ON(grpc_call_combiner_trace)) {
+            gpr_log(GPR_DEBUG,
+                    "call_combiner=%p: scheduling old cancel callback=%p",
+                    call_combiner, closure);
+          }
+          GRPC_CLOSURE_SCHED(exec_ctx, closure, GRPC_ERROR_NONE);
+        }
+        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..5cfb3f0c07
--- /dev/null
+++ b/src/core/lib/iomgr/call_combiner.h
@@ -0,0 +1,121 @@
+/*
+ *
+ * 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
+
+/// Registers \a closure to be invoked by \a call_combiner when
+/// grpc_call_combiner_cancel() is called.
+///
+/// Once a closure is registered, it will always be scheduled exactly
+/// once; this allows the closure to hold references that will be freed
+/// regardless of whether or not the call was cancelled.  If a cancellation
+/// does occur, the closure will be scheduled with the cancellation error;
+/// otherwise, it will be scheduled with GRPC_ERROR_NONE.
+///
+/// The closure will be scheduled in the following cases:
+/// - If grpc_call_combiner_cancel() was called prior to registering the
+///   closure, it will be scheduled immediately with the cancelation error.
+/// - If grpc_call_combiner_cancel() is called after registering the
+///   closure, the closure will be scheduled with the cancellation error.
+/// - If grpc_call_combiner_set_notify_on_cancel() is called again to
+///   register a new cancellation closure, the previous cancellation
+///   closure will be scheduled with GRPC_ERROR_NONE.
+///
+/// If \a closure is NULL, then no closure will be invoked on
+/// cancellation; this effectively unregisters the previously set closure.
+/// However, most filters will not need to explicitly unregister their
+/// callbacks, as this is done automatically when the call is destroyed.
+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..dd7dd44e79 100644
--- a/src/core/lib/security/transport/client_auth_filter.c
+++ b/src/core/lib/security/transport/client_auth_filter.c
@@ -39,6 +39,8 @@
 
 /* We can have a per-call credentials. */
 typedef struct {
+  grpc_call_stack *owning_call;
+  grpc_call_combiner *call_combiner;
   grpc_call_credentials *creds;
   bool have_host;
   bool have_method;
@@ -49,17 +51,12 @@ 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_result_closure;
+  grpc_closure check_call_host_cancel_closure;
+  grpc_closure get_request_metadata_cancel_closure;
 } call_data;
 
 /* We can have a per-channel credentials. */
@@ -68,43 +65,6 @@ 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) {
@@ -153,7 +113,8 @@ 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);
   }
 }
 
@@ -191,8 +152,12 @@ static void cancel_get_request_metadata(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;
-  grpc_call_credentials_cancel_get_request_metadata(
-      exec_ctx, calld->creds, &calld->md_array, GRPC_ERROR_REF(error));
+  if (error != GRPC_ERROR_NONE) {
+    grpc_call_credentials_cancel_get_request_metadata(
+        exec_ctx, calld->creds, &calld->md_array, GRPC_ERROR_REF(error));
+  }
+  GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call,
+                        "cancel_get_request_metadata");
 }
 
 static void send_security_metadata(grpc_exec_ctx *exec_ctx,
@@ -223,7 +188,8 @@ 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 +200,25 @@ 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_CALL_STACK_REF(calld->owning_call, "cancel_get_request_metadata");
+    grpc_call_combiner_set_notify_on_cancel(
+        exec_ctx, calld->call_combiner,
+        GRPC_CLOSURE_INIT(&calld->get_request_metadata_cancel_closure,
+                          cancel_get_request_metadata, elem,
+                          grpc_schedule_on_exec_ctx));
   }
 }
 
@@ -258,7 +227,6 @@ 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;
-
   if (error == GRPC_ERROR_NONE) {
     send_security_metadata(exec_ctx, elem, batch);
   } else {
@@ -271,7 +239,8 @@ 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);
   }
 }
@@ -281,9 +250,12 @@ static void cancel_check_call_host(grpc_exec_ctx *exec_ctx, void *arg,
   grpc_call_element *elem = (grpc_call_element *)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,
-      GRPC_ERROR_REF(error));
+  if (error != GRPC_ERROR_NONE) {
+    grpc_channel_security_connector_cancel_check_call_host(
+        exec_ctx, chand->security_connector, &calld->async_result_closure,
+        GRPC_ERROR_REF(error));
+  }
+  GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call, "cancel_check_call_host");
 }
 
 static void auth_start_transport_stream_op_batch(
@@ -295,52 +267,19 @@ 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 +304,27 @@ 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_CALL_STACK_REF(calld->owning_call, "cancel_check_call_host");
+        grpc_call_combiner_set_notify_on_cancel(
+            exec_ctx, calld->call_combiner,
+            GRPC_CLOSURE_INIT(&calld->check_call_host_cancel_closure,
+                              cancel_check_call_host, elem,
+                              grpc_schedule_on_exec_ctx));
       }
+      gpr_free(call_host);
       GPR_TIMER_END("auth_start_transport_stream_op_batch", 0);
       return; /* early exit */
     }
@@ -400,8 +340,8 @@ 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->owning_call = args->call_stack;
+  calld->call_combiner = args->call_combiner;
   return GRPC_ERROR_NONE;
 }
 
@@ -426,12 +366,6 @@ 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 +424,5 @@ 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..7f523c0883 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 +88,94 @@ 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;
   /* 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 (error != GRPC_ERROR_NONE &&
+      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));
+  }
+  GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call, "cancel_call");
+}
+
 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_CALL_STACK_REF(calld->owning_call, "cancel_call");
+      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 +209,8 @@ 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 +270,5 @@ 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..3aa20ffcd7 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");
 }
@@ -586,6 +598,12 @@ void grpc_call_unref(grpc_call *c) {
   if (cancel) {
     cancel_with_error(&exec_ctx, c, STATUS_FROM_API_OVERRIDE,
                       GRPC_ERROR_CANCELLED);
+  } else {
+    // Unset the call combiner cancellation closure.  This has the
+    // effect of scheduling the previously set cancellation closure, if
+    // any, so that it can release any internal references it may be
+    // holding to the call stack.
+    grpc_call_combiner_set_notify_on_cancel(&exec_ctx, &c->call_combiner, NULL);
   }
   GRPC_CALL_INTERNAL_UNREF(&exec_ctx, c, "destroy");
   grpc_exec_ctx_finish(&exec_ctx);
@@ -602,30 +620,37 @@ 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 +677,41 @@ 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 +1477,18 @@ 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 +1595,9 @@ 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 +1651,8 @@ 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 +1672,6 @@ 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 +1719,7 @@ 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 +1757,9 @@ 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 +1767,10 @@ 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 +1785,7 @@ 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 +1797,10 @@ 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 +1933,10 @@ 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 +1953,9 @@ 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 +2025,7 @@ 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 f8c84fb4a8..c525112496 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 0ca7688ca4..b32e8e7d4c 100644
--- a/src/core/lib/transport/transport.c
+++ b/src/core/lib/transport/transport.c
@@ -198,11 +198,6 @@ 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);
@@ -215,24 +210,24 @@ 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);