Merge pull request #2308 from ctiller/metadata-wants-to-be-debugged

Add metadata refcount debug helpers

10 files changed, 140 insertions, 53 deletions

diff --git a/src/core/transport/chttp2/hpack_parser.c b/src/core/transport/chttp2/hpack_parser.c
index c729e0f22f..f8bff42ed6 100644
--- a/src/core/transport/chttp2/hpack_parser.c
+++ b/src/core/transport/chttp2/hpack_parser.c
@@ -622,7 +622,7 @@ static const gpr_uint8 inverse_base64[256] = {
 static void on_hdr(grpc_chttp2_hpack_parser *p, grpc_mdelem *md,
                    int add_to_table) {
   if (add_to_table) {
-    grpc_mdelem_ref(md);
+    GRPC_MDELEM_REF(md);
     grpc_chttp2_hptbl_add(&p->table, md);
   }
   p->on_header(p->on_header_user_data, md);
@@ -711,7 +711,7 @@ static int parse_stream_dep0(grpc_chttp2_hpack_parser *p, const gpr_uint8 *cur,
 static int finish_indexed_field(grpc_chttp2_hpack_parser *p,
                                 const gpr_uint8 *cur, const gpr_uint8 *end) {
   grpc_mdelem *md = grpc_chttp2_hptbl_lookup(&p->table, p->index);
-  grpc_mdelem_ref(md);
+  GRPC_MDELEM_REF(md);
   on_hdr(p, md, 0);
   return parse_begin(p, cur, end);
 }
@@ -740,7 +740,7 @@ static int finish_lithdr_incidx(grpc_chttp2_hpack_parser *p,
                                 const gpr_uint8 *cur, const gpr_uint8 *end) {
   grpc_mdelem *md = grpc_chttp2_hptbl_lookup(&p->table, p->index);
   on_hdr(p, grpc_mdelem_from_metadata_strings(p->table.mdctx,
-                                              grpc_mdstr_ref(md->key),
+                                              GRPC_MDSTR_REF(md->key),
                                               take_string(p, &p->value)),
          1);
   return parse_begin(p, cur, end);
@@ -793,7 +793,7 @@ static int finish_lithdr_notidx(grpc_chttp2_hpack_parser *p,
                                 const gpr_uint8 *cur, const gpr_uint8 *end) {
   grpc_mdelem *md = grpc_chttp2_hptbl_lookup(&p->table, p->index);
   on_hdr(p, grpc_mdelem_from_metadata_strings(p->table.mdctx,
-                                              grpc_mdstr_ref(md->key),
+                                              GRPC_MDSTR_REF(md->key),
                                               take_string(p, &p->value)),
          0);
   return parse_begin(p, cur, end);
@@ -846,7 +846,7 @@ static int finish_lithdr_nvridx(grpc_chttp2_hpack_parser *p,
                                 const gpr_uint8 *cur, const gpr_uint8 *end) {
   grpc_mdelem *md = grpc_chttp2_hptbl_lookup(&p->table, p->index);
   on_hdr(p, grpc_mdelem_from_metadata_strings(p->table.mdctx,
-                                              grpc_mdstr_ref(md->key),
+                                              GRPC_MDSTR_REF(md->key),
                                               take_string(p, &p->value)),
          0);
   return parse_begin(p, cur, end);
@@ -1336,7 +1336,7 @@ static void on_header_not_set(void *user_data, grpc_mdelem *md) {
           valuehex);
   gpr_free(keyhex);
   gpr_free(valuehex);
-  grpc_mdelem_unref(md);
+  GRPC_MDELEM_UNREF(md);
   abort();
 }
 
diff --git a/src/core/transport/chttp2/hpack_table.c b/src/core/transport/chttp2/hpack_table.c
index 372e71d68f..4fc154380e 100644
--- a/src/core/transport/chttp2/hpack_table.c
+++ b/src/core/transport/chttp2/hpack_table.c
@@ -122,10 +122,10 @@ void grpc_chttp2_hptbl_init(grpc_chttp2_hptbl *tbl, grpc_mdctx *mdctx) {
 void grpc_chttp2_hptbl_destroy(grpc_chttp2_hptbl *tbl) {
   size_t i;
   for (i = 0; i < GRPC_CHTTP2_LAST_STATIC_ENTRY; i++) {
-    grpc_mdelem_unref(tbl->static_ents[i]);
+    GRPC_MDELEM_UNREF(tbl->static_ents[i]);
   }
   for (i = 0; i < tbl->num_ents; i++) {
-    grpc_mdelem_unref(
+    GRPC_MDELEM_UNREF(
         tbl->ents[(tbl->first_ent + i) % GRPC_CHTTP2_MAX_TABLE_COUNT]);
   }
 }
@@ -155,7 +155,7 @@ static void evict1(grpc_chttp2_hptbl *tbl) {
                    GRPC_CHTTP2_HPACK_ENTRY_OVERHEAD;
   tbl->first_ent = (tbl->first_ent + 1) % GRPC_CHTTP2_MAX_TABLE_COUNT;
   tbl->num_ents--;
-  grpc_mdelem_unref(first_ent);
+  GRPC_MDELEM_UNREF(first_ent);
 }
 
 void grpc_chttp2_hptbl_add(grpc_chttp2_hptbl *tbl, grpc_mdelem *md) {
diff --git a/src/core/transport/chttp2/incoming_metadata.c b/src/core/transport/chttp2/incoming_metadata.c
index 68e0912b9c..77162a6864 100644
--- a/src/core/transport/chttp2/incoming_metadata.c
+++ b/src/core/transport/chttp2/incoming_metadata.c
@@ -49,7 +49,7 @@ void grpc_chttp2_incoming_metadata_buffer_destroy(
     grpc_chttp2_incoming_metadata_buffer *buffer) {
   size_t i;
   for (i = 0; i < buffer->count; i++) {
-    grpc_mdelem_unref(buffer->elems[i].md);
+    GRPC_MDELEM_UNREF(buffer->elems[i].md);
   }
   gpr_free(buffer->elems);
 }
diff --git a/src/core/transport/chttp2/parsing.c b/src/core/transport/chttp2/parsing.c
index 4664a0895c..130167f830 100644
--- a/src/core/transport/chttp2/parsing.c
+++ b/src/core/transport/chttp2/parsing.c
@@ -463,7 +463,7 @@ static grpc_chttp2_parse_error skip_parser(
   return GRPC_CHTTP2_PARSE_OK;
 }
 
-static void skip_header(void *tp, grpc_mdelem *md) { grpc_mdelem_unref(md); }
+static void skip_header(void *tp, grpc_mdelem *md) { GRPC_MDELEM_UNREF(md); }
 
 static int init_skip_frame_parser(
     grpc_chttp2_transport_parsing *transport_parsing, int is_header) {
@@ -600,7 +600,7 @@ static void on_header(void *tp, grpc_mdelem *md) {
     grpc_chttp2_incoming_metadata_buffer_set_deadline(
         &stream_parsing->incoming_metadata,
         gpr_time_add(gpr_now(), *cached_timeout));
-    grpc_mdelem_unref(md);
+    GRPC_MDELEM_UNREF(md);
   } else {
     grpc_chttp2_incoming_metadata_buffer_add(&stream_parsing->incoming_metadata,
                                              md);
diff --git a/src/core/transport/chttp2/stream_encoder.c b/src/core/transport/chttp2/stream_encoder.c
index cf78ac50cc..56ab82006a 100644
--- a/src/core/transport/chttp2/stream_encoder.c
+++ b/src/core/transport/chttp2/stream_encoder.c
@@ -247,19 +247,19 @@ static grpc_mdelem *add_elem(grpc_chttp2_hpack_compressor *c,
   } else if (c->entries_keys[HASH_FRAGMENT_3(key_hash)] == elem->key) {
     c->indices_keys[HASH_FRAGMENT_3(key_hash)] = new_index;
   } else if (c->entries_keys[HASH_FRAGMENT_2(key_hash)] == NULL) {
-    c->entries_keys[HASH_FRAGMENT_2(key_hash)] = grpc_mdstr_ref(elem->key);
+    c->entries_keys[HASH_FRAGMENT_2(key_hash)] = GRPC_MDSTR_REF(elem->key);
     c->indices_keys[HASH_FRAGMENT_2(key_hash)] = new_index;
   } else if (c->entries_keys[HASH_FRAGMENT_3(key_hash)] == NULL) {
-    c->entries_keys[HASH_FRAGMENT_3(key_hash)] = grpc_mdstr_ref(elem->key);
+    c->entries_keys[HASH_FRAGMENT_3(key_hash)] = GRPC_MDSTR_REF(elem->key);
     c->indices_keys[HASH_FRAGMENT_3(key_hash)] = new_index;
   } else if (c->indices_keys[HASH_FRAGMENT_2(key_hash)] <
              c->indices_keys[HASH_FRAGMENT_3(key_hash)]) {
-    grpc_mdstr_unref(c->entries_keys[HASH_FRAGMENT_2(key_hash)]);
-    c->entries_keys[HASH_FRAGMENT_2(key_hash)] = grpc_mdstr_ref(elem->key);
+    GRPC_MDSTR_UNREF(c->entries_keys[HASH_FRAGMENT_2(key_hash)]);
+    c->entries_keys[HASH_FRAGMENT_2(key_hash)] = GRPC_MDSTR_REF(elem->key);
     c->indices_keys[HASH_FRAGMENT_2(key_hash)] = new_index;
   } else {
-    grpc_mdstr_unref(c->entries_keys[HASH_FRAGMENT_3(key_hash)]);
-    c->entries_keys[HASH_FRAGMENT_3(key_hash)] = grpc_mdstr_ref(elem->key);
+    GRPC_MDSTR_UNREF(c->entries_keys[HASH_FRAGMENT_3(key_hash)]);
+    c->entries_keys[HASH_FRAGMENT_3(key_hash)] = GRPC_MDSTR_REF(elem->key);
     c->indices_keys[HASH_FRAGMENT_3(key_hash)] = new_index;
   }
 
@@ -439,10 +439,10 @@ static void deadline_enc(grpc_chttp2_hpack_compressor *c, gpr_timespec deadline,
   grpc_mdelem *mdelem;
   grpc_chttp2_encode_timeout(gpr_time_sub(deadline, gpr_now()), timeout_str);
   mdelem = grpc_mdelem_from_metadata_strings(
-      c->mdctx, grpc_mdstr_ref(c->timeout_key_str),
+      c->mdctx, GRPC_MDSTR_REF(c->timeout_key_str),
       grpc_mdstr_from_string(c->mdctx, timeout_str));
   mdelem = hpack_enc(c, mdelem, st);
-  if (mdelem) grpc_mdelem_unref(mdelem);
+  if (mdelem) GRPC_MDELEM_UNREF(mdelem);
 }
 
 gpr_slice grpc_chttp2_data_frame_create_empty_close(gpr_uint32 id) {
@@ -461,10 +461,10 @@ void grpc_chttp2_hpack_compressor_init(grpc_chttp2_hpack_compressor *c,
 void grpc_chttp2_hpack_compressor_destroy(grpc_chttp2_hpack_compressor *c) {
   int i;
   for (i = 0; i < GRPC_CHTTP2_HPACKC_NUM_VALUES; i++) {
-    if (c->entries_keys[i]) grpc_mdstr_unref(c->entries_keys[i]);
-    if (c->entries_elems[i]) grpc_mdelem_unref(c->entries_elems[i]);
+    if (c->entries_keys[i]) GRPC_MDSTR_UNREF(c->entries_keys[i]);
+    if (c->entries_elems[i]) GRPC_MDELEM_UNREF(c->entries_elems[i]);
   }
-  grpc_mdstr_unref(c->timeout_key_str);
+  GRPC_MDSTR_UNREF(c->timeout_key_str);
 }
 
 gpr_uint32 grpc_chttp2_preencode(grpc_stream_op *inops, size_t *inops_count,
@@ -620,10 +620,10 @@ void grpc_chttp2_encode(grpc_stream_op *ops, size_t ops_count, int eof,
       op = &ops[unref_op];
       if (op->type != GRPC_OP_METADATA) continue;
       for (l = op->data.metadata.list.head; l; l = l->next) {
-        if (l->md) grpc_mdctx_locked_mdelem_unref(mdctx, l->md);
+        if (l->md) GRPC_MDCTX_LOCKED_MDELEM_UNREF(mdctx, l->md);
       }
       for (l = op->data.metadata.garbage.head; l; l = l->next) {
-        grpc_mdctx_locked_mdelem_unref(mdctx, l->md);
+        GRPC_MDCTX_LOCKED_MDELEM_UNREF(mdctx, l->md);
       }
     }
     grpc_mdctx_unlock(mdctx);
diff --git a/src/core/transport/chttp2_transport.c b/src/core/transport/chttp2_transport.c
index 47d0c548cb..322ff39820 100644
--- a/src/core/transport/chttp2_transport.c
+++ b/src/core/transport/chttp2_transport.c
@@ -139,7 +139,7 @@ static void destruct_transport(grpc_chttp2_transport *t) {
   grpc_chttp2_hpack_parser_destroy(&t->parsing.hpack_parser);
   grpc_chttp2_goaway_parser_destroy(&t->parsing.goaway_parser);
 
-  grpc_mdstr_unref(t->parsing.str_grpc_timeout);
+  GRPC_MDSTR_UNREF(t->parsing.str_grpc_timeout);
 
   for (i = 0; i < STREAM_LIST_COUNT; i++) {
     GPR_ASSERT(t->lists[i].head == NULL);
diff --git a/src/core/transport/metadata.c b/src/core/transport/metadata.c
index c80d67823f..9cbb0952d0 100644
--- a/src/core/transport/metadata.c
+++ b/src/core/transport/metadata.c
@@ -48,6 +48,20 @@
 #define INITIAL_STRTAB_CAPACITY 4
 #define INITIAL_MDTAB_CAPACITY 4
 
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+#define DEBUG_ARGS , const char *file, int line
+#define FWD_DEBUG_ARGS , file, line
+#define INTERNAL_STRING_REF(s) internal_string_ref((s), __FILE__, __LINE__)
+#define INTERNAL_STRING_UNREF(s) internal_string_unref((s), __FILE__, __LINE__)
+#define REF_MD_LOCKED(s) ref_md_locked((s), __FILE__, __LINE__)
+#else
+#define DEBUG_ARGS
+#define FWD_DEBUG_ARGS
+#define INTERNAL_STRING_REF(s) internal_string_ref((s))
+#define INTERNAL_STRING_UNREF(s) internal_string_unref((s))
+#define REF_MD_LOCKED(s) ref_md_locked((s))
+#endif
+
 typedef struct internal_string {
   /* must be byte compatible with grpc_mdstr */
   gpr_slice slice;
@@ -96,8 +110,8 @@ struct grpc_mdctx {
   size_t mdtab_capacity;
 };
 
-static void internal_string_ref(internal_string *s);
-static void internal_string_unref(internal_string *s);
+static void internal_string_ref(internal_string *s DEBUG_ARGS);
+static void internal_string_unref(internal_string *s DEBUG_ARGS);
 static void discard_metadata(grpc_mdctx *ctx);
 static void gc_mdtab(grpc_mdctx *ctx);
 static void metadata_context_destroy_locked(grpc_mdctx *ctx);
@@ -132,7 +146,15 @@ static void unlock(grpc_mdctx *ctx) {
   gpr_mu_unlock(&ctx->mu);
 }
 
-static void ref_md_locked(internal_metadata *md) {
+static void ref_md_locked(internal_metadata *md DEBUG_ARGS) {
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
+          "ELM   REF:%p:%d->%d: '%s' = '%s'", md,
+          gpr_atm_no_barrier_load(&md->refcnt),
+          gpr_atm_no_barrier_load(&md->refcnt) + 1,
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
+#endif
   if (0 == gpr_atm_no_barrier_fetch_add(&md->refcnt, 1)) {
     md->context->mdtab_free--;
   }
@@ -173,8 +195,8 @@ static void discard_metadata(grpc_mdctx *ctx) {
     while (cur) {
       GPR_ASSERT(gpr_atm_acq_load(&cur->refcnt) == 0);
       next = cur->bucket_next;
-      internal_string_unref(cur->key);
-      internal_string_unref(cur->value);
+      INTERNAL_STRING_UNREF(cur->key);
+      INTERNAL_STRING_UNREF(cur->value);
       if (cur->user_data) {
         cur->destroy_user_data(cur->user_data);
       }
@@ -248,9 +270,19 @@ static void internal_destroy_string(internal_string *is) {
   gpr_free(is);
 }
 
-static void internal_string_ref(internal_string *s) { ++s->refs; }
+static void internal_string_ref(internal_string *s DEBUG_ARGS) {
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG, "STR   REF:%p:%d->%d: '%s'", s,
+          s->refs, s->refs + 1, grpc_mdstr_as_c_string((grpc_mdstr *)s));
+#endif
+  ++s->refs;
+}
 
-static void internal_string_unref(internal_string *s) {
+static void internal_string_unref(internal_string *s DEBUG_ARGS) {
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG, "STR UNREF:%p:%d->%d: '%s'", s,
+          s->refs, s->refs - 1, grpc_mdstr_as_c_string((grpc_mdstr *)s));
+#endif
   GPR_ASSERT(s->refs > 0);
   if (0 == --s->refs) {
     internal_destroy_string(s);
@@ -262,7 +294,7 @@ static void slice_ref(void *p) {
       (internal_string *)((char *)p - offsetof(internal_string, refcount));
   grpc_mdctx *ctx = is->context;
   lock(ctx);
-  internal_string_ref(is);
+  INTERNAL_STRING_REF(is);
   unlock(ctx);
 }
 
@@ -271,7 +303,7 @@ static void slice_unref(void *p) {
       (internal_string *)((char *)p - offsetof(internal_string, refcount));
   grpc_mdctx *ctx = is->context;
   lock(ctx);
-  internal_string_unref(is);
+  INTERNAL_STRING_UNREF(is);
   unlock(ctx);
 }
 
@@ -297,7 +329,7 @@ grpc_mdstr *grpc_mdstr_from_buffer(grpc_mdctx *ctx, const gpr_uint8 *buf,
   for (s = ctx->strtab[hash % ctx->strtab_capacity]; s; s = s->bucket_next) {
     if (s->hash == hash && GPR_SLICE_LENGTH(s->slice) == length &&
         0 == memcmp(buf, GPR_SLICE_START_PTR(s->slice), length)) {
-      internal_string_ref(s);
+      INTERNAL_STRING_REF(s);
       unlock(ctx);
       return (grpc_mdstr *)s;
     }
@@ -353,8 +385,8 @@ static void gc_mdtab(grpc_mdctx *ctx) {
     for (md = ctx->mdtab[i]; md; md = next) {
       next = md->bucket_next;
       if (gpr_atm_acq_load(&md->refcnt) == 0) {
-        internal_string_unref(md->key);
-        internal_string_unref(md->value);
+        INTERNAL_STRING_UNREF(md->key);
+        INTERNAL_STRING_UNREF(md->value);
         if (md->user_data) {
           md->destroy_user_data(md->user_data);
         }
@@ -418,9 +450,9 @@ grpc_mdelem *grpc_mdelem_from_metadata_strings(grpc_mdctx *ctx,
   /* search for an existing pair */
   for (md = ctx->mdtab[hash % ctx->mdtab_capacity]; md; md = md->bucket_next) {
     if (md->key == key && md->value == value) {
-      ref_md_locked(md);
-      internal_string_unref(key);
-      internal_string_unref(value);
+      REF_MD_LOCKED(md);
+      INTERNAL_STRING_UNREF(key);
+      INTERNAL_STRING_UNREF(value);
       unlock(ctx);
       return (grpc_mdelem *)md;
     }
@@ -435,6 +467,12 @@ grpc_mdelem *grpc_mdelem_from_metadata_strings(grpc_mdctx *ctx,
   md->user_data = NULL;
   md->destroy_user_data = NULL;
   md->bucket_next = ctx->mdtab[hash % ctx->mdtab_capacity];
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+  gpr_log(GPR_DEBUG, "ELM   NEW:%p:%d: '%s' = '%s'", md,
+          gpr_atm_no_barrier_load(&md->refcnt),
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
+#endif
   ctx->mdtab[hash % ctx->mdtab_capacity] = md;
   ctx->mdtab_count++;
 
@@ -469,8 +507,16 @@ grpc_mdelem *grpc_mdelem_from_string_and_buffer(grpc_mdctx *ctx,
       grpc_mdstr_from_buffer(ctx, value, value_length));
 }
 
-grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *gmd) {
+grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *gmd DEBUG_ARGS) {
   internal_metadata *md = (internal_metadata *)gmd;
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
+          "ELM   REF:%p:%d->%d: '%s' = '%s'", md,
+          gpr_atm_no_barrier_load(&md->refcnt),
+          gpr_atm_no_barrier_load(&md->refcnt) + 1,
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
+#endif
   /* we can assume the ref count is >= 1 as the application is calling
      this function - meaning that no adjustment to mdtab_free is necessary,
      simplifying the logic here to be just an atomic increment */
@@ -480,10 +526,18 @@ grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *gmd) {
   return gmd;
 }
 
-void grpc_mdelem_unref(grpc_mdelem *gmd) {
+void grpc_mdelem_unref(grpc_mdelem *gmd DEBUG_ARGS) {
   internal_metadata *md = (internal_metadata *)gmd;
   grpc_mdctx *ctx = md->context;
   lock(ctx);
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
+          "ELM UNREF:%p:%d->%d: '%s' = '%s'", md,
+          gpr_atm_no_barrier_load(&md->refcnt),
+          gpr_atm_no_barrier_load(&md->refcnt) - 1,
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
+#endif
   assert(gpr_atm_no_barrier_load(&md->refcnt) >= 1);
   if (1 == gpr_atm_full_fetch_add(&md->refcnt, -1)) {
     ctx->mdtab_free++;
@@ -495,20 +549,20 @@ const char *grpc_mdstr_as_c_string(grpc_mdstr *s) {
   return (const char *)GPR_SLICE_START_PTR(s->slice);
 }
 
-grpc_mdstr *grpc_mdstr_ref(grpc_mdstr *gs) {
+grpc_mdstr *grpc_mdstr_ref(grpc_mdstr *gs DEBUG_ARGS) {
   internal_string *s = (internal_string *)gs;
   grpc_mdctx *ctx = s->context;
   lock(ctx);
-  internal_string_ref(s);
+  internal_string_ref(s FWD_DEBUG_ARGS);
   unlock(ctx);
   return gs;
 }
 
-void grpc_mdstr_unref(grpc_mdstr *gs) {
+void grpc_mdstr_unref(grpc_mdstr *gs DEBUG_ARGS) {
   internal_string *s = (internal_string *)gs;
   grpc_mdctx *ctx = s->context;
   lock(ctx);
-  internal_string_unref(s);
+  internal_string_unref(s FWD_DEBUG_ARGS);
   unlock(ctx);
 }
 
@@ -558,10 +612,19 @@ gpr_slice grpc_mdstr_as_base64_encoded_and_huffman_compressed(grpc_mdstr *gs) {
 
 void grpc_mdctx_lock(grpc_mdctx *ctx) { lock(ctx); }
 
-void grpc_mdctx_locked_mdelem_unref(grpc_mdctx *ctx, grpc_mdelem *gmd) {
+void grpc_mdctx_locked_mdelem_unref(grpc_mdctx *ctx,
+                                    grpc_mdelem *gmd DEBUG_ARGS) {
   internal_metadata *md = (internal_metadata *)gmd;
   grpc_mdctx *elem_ctx = md->context;
   GPR_ASSERT(ctx == elem_ctx);
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+  gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
+          "ELM UNREF:%p:%d->%d: '%s' = '%s'", md,
+          gpr_atm_no_barrier_load(&md->refcnt),
+          gpr_atm_no_barrier_load(&md->refcnt) - 1,
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->key),
+          grpc_mdstr_as_c_string((grpc_mdstr *)md->value));
+#endif
   assert(gpr_atm_no_barrier_load(&md->refcnt) >= 1);
   if (1 == gpr_atm_full_fetch_add(&md->refcnt, -1)) {
     ctx->mdtab_free++;
diff --git a/src/core/transport/metadata.h b/src/core/transport/metadata.h
index 76e3f3c1f8..99b15322c3 100644
--- a/src/core/transport/metadata.h
+++ b/src/core/transport/metadata.h
@@ -127,11 +127,25 @@ void grpc_mdelem_set_user_data(grpc_mdelem *md, void (*destroy_func)(void *),
                                void *user_data);
 
 /* Reference counting */
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+#define GRPC_MDSTR_REF(s) grpc_mdstr_ref((s), __FILE__, __LINE__)
+#define GRPC_MDSTR_UNREF(s) grpc_mdstr_unref((s), __FILE__, __LINE__)
+#define GRPC_MDELEM_REF(s) grpc_mdelem_ref((s), __FILE__, __LINE__)
+#define GRPC_MDELEM_UNREF(s) grpc_mdelem_unref((s), __FILE__, __LINE__)
+grpc_mdstr *grpc_mdstr_ref(grpc_mdstr *s, const char *file, int line);
+void grpc_mdstr_unref(grpc_mdstr *s, const char *file, int line);
+grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *md, const char *file, int line);
+void grpc_mdelem_unref(grpc_mdelem *md, const char *file, int line);
+#else
+#define GRPC_MDSTR_REF(s) grpc_mdstr_ref((s))
+#define GRPC_MDSTR_UNREF(s) grpc_mdstr_unref((s))
+#define GRPC_MDELEM_REF(s) grpc_mdelem_ref((s))
+#define GRPC_MDELEM_UNREF(s) grpc_mdelem_unref((s))
 grpc_mdstr *grpc_mdstr_ref(grpc_mdstr *s);
 void grpc_mdstr_unref(grpc_mdstr *s);
-
 grpc_mdelem *grpc_mdelem_ref(grpc_mdelem *md);
 void grpc_mdelem_unref(grpc_mdelem *md);
+#endif
 
 /* Recover a char* from a grpc_mdstr. The returned string is null terminated.
    Does not promise that the returned string has no embedded nulls however. */
@@ -147,8 +161,18 @@ int grpc_mdstr_is_bin_suffixed(grpc_mdstr *s);
 /* Lock the metadata context: it's only safe to call _locked_ functions against
    this context from the calling thread until grpc_mdctx_unlock is called */
 void grpc_mdctx_lock(grpc_mdctx *ctx);
+#ifdef GRPC_METADATA_REFCOUNT_DEBUG
+#define GRPC_MDCTX_LOCKED_MDELEM_UNREF(ctx, elem) \
+  grpc_mdctx_locked_mdelem_unref((ctx), (elem), __FILE__, __LINE__)
+/* Unref a metadata element */
+void grpc_mdctx_locked_mdelem_unref(grpc_mdctx *ctx, grpc_mdelem *elem,
+                                    const char *file, int line);
+#else
+#define GRPC_MDCTX_LOCKED_MDELEM_UNREF(ctx, elem) \
+  grpc_mdctx_locked_mdelem_unref((ctx), (elem))
 /* Unref a metadata element */
 void grpc_mdctx_locked_mdelem_unref(grpc_mdctx *ctx, grpc_mdelem *elem);
+#endif
 /* Unlock the metadata context */
 void grpc_mdctx_unlock(grpc_mdctx *ctx);
 
diff --git a/src/core/transport/stream_op.c b/src/core/transport/stream_op.c
index 81df5455f6..fdb50c6b71 100644
--- a/src/core/transport/stream_op.c
+++ b/src/core/transport/stream_op.c
@@ -211,10 +211,10 @@ void grpc_metadata_batch_init(grpc_metadata_batch *batch) {
 void grpc_metadata_batch_destroy(grpc_metadata_batch *batch) {
   grpc_linked_mdelem *l;
   for (l = batch->list.head; l; l = l->next) {
-    grpc_mdelem_unref(l->md);
+    GRPC_MDELEM_UNREF(l->md);
   }
   for (l = batch->garbage.head; l; l = l->next) {
-    grpc_mdelem_unref(l->md);
+    GRPC_MDELEM_UNREF(l->md);
   }
 }
 
@@ -315,7 +315,7 @@ void grpc_metadata_batch_filter(grpc_metadata_batch *batch,
       assert_valid_list(&batch->list);
       link_head(&batch->garbage, l);
     } else if (filt != orig) {
-      grpc_mdelem_unref(orig);
+      GRPC_MDELEM_UNREF(orig);
       l->md = filt;
     }
   }
diff --git a/src/core/transport/transport.c b/src/core/transport/transport.c
index fe565944ed..2689e3028a 100644
--- a/src/core/transport/transport.c
+++ b/src/core/transport/transport.c
@@ -85,6 +85,6 @@ void grpc_transport_stream_op_add_cancellation(grpc_transport_stream_op *op,
     op->cancel_with_status = status;
   }
   if (message) {
-    grpc_mdstr_unref(message);
+    GRPC_MDSTR_UNREF(message);
   }
 }