Remove extraneous locks on cq checks

src/core/lib/surface/completion_queue.c

@@ -71,6 +71,9 @@ struct grpc_completion_queue {
   gpr_refcount pending_events;
   /** Once owning_refs drops to zero, we will destroy the cq */
   gpr_refcount owning_refs;
+  /** counter of how many things have ever been queued on this completion queue
+      useful for avoiding locks to check the queue */
+  gpr_atm things_queued_ever;
   /** 0 initially, 1 once we've begun shutting down */
   int shutdown;
   int shutdown_called;
@@ -125,15 +128,6 @@ void grpc_cq_global_shutdown(void) {
   }
 }
 
-struct grpc_cq_alarm {
-  grpc_timer alarm;
-  grpc_cq_completion completion;
-  /** completion queue where events about this alarm will be posted */
-  grpc_completion_queue *cq;
-  /** user supplied tag */
-  void *tag;
-};
-
 grpc_completion_queue *grpc_completion_queue_create(void *reserved) {
   grpc_completion_queue *cc;
   GPR_ASSERT(!reserved);
@@ -170,6 +164,7 @@ grpc_completion_queue *grpc_completion_queue_create(void *reserved) {
   cc->is_server_cq = 0;
   cc->is_non_listening_server_cq = 0;
   cc->num_pluckers = 0;
+  gpr_atm_no_barrier_store(&cc->things_queued_ever, 0);
 #ifndef NDEBUG
   cc->outstanding_tag_count = 0;
 #endif
@@ -280,6 +275,7 @@ void grpc_cq_end_op(grpc_exec_ctx *exec_ctx, grpc_completion_queue *cc,
   GPR_ASSERT(found);
 #endif
   shutdown = gpr_unref(&cc->pending_events);
+  gpr_atm_no_barrier_fetch_add(&cc->things_queued_ever, 1);
   if (!shutdown) {
     cc->completed_tail->next =
         ((uintptr_t)storage) | (1u & (uintptr_t)cc->completed_tail->next);
@@ -318,6 +314,7 @@ void grpc_cq_end_op(grpc_exec_ctx *exec_ctx, grpc_completion_queue *cc,
 }
 
 typedef struct {
+  gpr_atm last_seen_things_queued_ever;
   grpc_completion_queue *cq;
   gpr_timespec deadline;
   grpc_cq_completion *stolen_completion;
@@ -328,17 +325,23 @@ static bool cq_is_next_finished(grpc_exec_ctx *exec_ctx, void *arg) {
   cq_is_finished_arg *a = arg;
   grpc_completion_queue *cq = a->cq;
   GPR_ASSERT(a->stolen_completion == NULL);
-  gpr_mu_lock(cq->mu);
-  if (cq->completed_tail != &cq->completed_head) {
-    a->stolen_completion = (grpc_cq_completion *)cq->completed_head.next;
-    cq->completed_head.next = a->stolen_completion->next & ~(uintptr_t)1;
-    if (a->stolen_completion == cq->completed_tail) {
-      cq->completed_tail = &cq->completed_head;
+  gpr_atm current_last_seen_things_queued_ever =
+      gpr_atm_no_barrier_load(&cq->things_queued_ever);
+  if (current_last_seen_things_queued_ever != a->last_seen_things_queued_ever) {
+    gpr_mu_lock(cq->mu);
+    a->last_seen_things_queued_ever =
+        gpr_atm_no_barrier_load(&cq->things_queued_ever);
+    if (cq->completed_tail != &cq->completed_head) {
+      a->stolen_completion = (grpc_cq_completion *)cq->completed_head.next;
+      cq->completed_head.next = a->stolen_completion->next & ~(uintptr_t)1;
+      if (a->stolen_completion == cq->completed_tail) {
+        cq->completed_tail = &cq->completed_head;
+      }
+      gpr_mu_unlock(cq->mu);
+      return true;
     }
     gpr_mu_unlock(cq->mu);
-    return true;
   }
-  gpr_mu_unlock(cq->mu);
   return gpr_time_cmp(a->deadline, gpr_now(a->deadline.clock_type)) < 0;
 }
 
@@ -386,12 +389,13 @@ grpc_event grpc_completion_queue_next(grpc_completion_queue *cc,
 
   deadline = gpr_convert_clock_type(deadline, GPR_CLOCK_MONOTONIC);
 
-  cq_is_finished_arg is_finished_arg = {cc, deadline, NULL, NULL};
-  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT_WITH_FINISH_CHECK(
-      cq_is_next_finished, &is_finished_arg);
-
   GRPC_CQ_INTERNAL_REF(cc, "next");
   gpr_mu_lock(cc->mu);
+  cq_is_finished_arg is_finished_arg = {
+      gpr_atm_no_barrier_load(&cc->things_queued_ever), cc, deadline, NULL,
+      NULL};
+  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT_WITH_FINISH_CHECK(
+      cq_is_next_finished, &is_finished_arg);
   for (;;) {
     if (is_finished_arg.stolen_completion != NULL) {
       gpr_mu_unlock(cc->mu);
@@ -496,23 +500,29 @@ static bool cq_is_pluck_finished(grpc_exec_ctx *exec_ctx, void *arg) {
   cq_is_finished_arg *a = arg;
   grpc_completion_queue *cq = a->cq;
   GPR_ASSERT(a->stolen_completion == NULL);
-  gpr_mu_lock(cq->mu);
-  grpc_cq_completion *c;
-  grpc_cq_completion *prev = &cq->completed_head;
-  while ((c = (grpc_cq_completion *)(prev->next & ~(uintptr_t)1)) !=
-         &cq->completed_head) {
-    if (c->tag == a->tag) {
-      prev->next = (prev->next & (uintptr_t)1) | (c->next & ~(uintptr_t)1);
-      if (c == cq->completed_tail) {
-        cq->completed_tail = prev;
+  gpr_atm current_last_seen_things_queued_ever =
+      gpr_atm_no_barrier_load(&cq->things_queued_ever);
+  if (current_last_seen_things_queued_ever != a->last_seen_things_queued_ever) {
+    gpr_mu_lock(cq->mu);
+    a->last_seen_things_queued_ever =
+        gpr_atm_no_barrier_load(&cq->things_queued_ever);
+    grpc_cq_completion *c;
+    grpc_cq_completion *prev = &cq->completed_head;
+    while ((c = (grpc_cq_completion *)(prev->next & ~(uintptr_t)1)) !=
+           &cq->completed_head) {
+      if (c->tag == a->tag) {
+        prev->next = (prev->next & (uintptr_t)1) | (c->next & ~(uintptr_t)1);
+        if (c == cq->completed_tail) {
+          cq->completed_tail = prev;
+        }
+        gpr_mu_unlock(cq->mu);
+        a->stolen_completion = c;
+        return true;
       }
-      gpr_mu_unlock(cq->mu);
-      a->stolen_completion = c;
-      return true;
+      prev = c;
     }
-    prev = c;
+    gpr_mu_unlock(cq->mu);
   }
-  gpr_mu_unlock(cq->mu);
   return gpr_time_cmp(a->deadline, gpr_now(a->deadline.clock_type)) < 0;
 }
 
@@ -543,12 +553,13 @@ grpc_event grpc_completion_queue_pluck(grpc_completion_queue *cc, void *tag,
 
   deadline = gpr_convert_clock_type(deadline, GPR_CLOCK_MONOTONIC);
 
-  cq_is_finished_arg is_finished_arg = {cc, deadline, NULL, tag};
-  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT_WITH_FINISH_CHECK(
-      cq_is_pluck_finished, &is_finished_arg);
-
   GRPC_CQ_INTERNAL_REF(cc, "pluck");
   gpr_mu_lock(cc->mu);
+  cq_is_finished_arg is_finished_arg = {
+      gpr_atm_no_barrier_load(&cc->things_queued_ever), cc, deadline, NULL,
+      tag};
+  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT_WITH_FINISH_CHECK(
+      cq_is_pluck_finished, &is_finished_arg);
   for (;;) {
     if (is_finished_arg.stolen_completion != NULL) {
       gpr_mu_unlock(cc->mu);