1 files changed, 19 insertions, 4 deletions
diff --git a/src/core/lib/iomgr/combiner.c b/src/core/lib/iomgr/combiner.c
index 38eace12c7..b77a68eead 100644
--- a/src/core/lib/iomgr/combiner.c
+++ b/src/core/lib/iomgr/combiner.c
@@ -59,6 +59,11 @@ struct grpc_combiner {
   grpc_closure_scheduler scheduler;
   grpc_closure_scheduler finally_scheduler;
   gpr_mpscq queue;
+  // either:
+  // a pointer to the initiating exec ctx if that is the only exec_ctx that has
+  // ever queued to this combiner, or NULL. If this is non-null, it's not
+  // dereferencable (since the initiating exec_ctx may have gone out of scope)
+  gpr_atm initiating_exec_ctx_or_null;
   // state is:
   // lower bit - zero if orphaned (STATE_UNORPHANED)
   // other bits - number of items queued on the lock (STATE_ELEM_COUNT_LOW_BIT)
@@ -168,15 +173,25 @@ static void combiner_exec(grpc_exec_ctx *exec_ctx, grpc_closure *cl,
   GRPC_COMBINER_TRACE(gpr_log(GPR_DEBUG,
                               "C:%p grpc_combiner_execute c=%p last=%" PRIdPTR,
                               lock, cl, last));
-  GPR_ASSERT(last & STATE_UNORPHANED);  // ensure lock has not been destroyed
-  assert(cl->cb);
-  cl->error_data.error = error;
-  gpr_mpscq_push(&lock->queue, &cl->next_data.atm_next);
   if (last == 1) {
+    gpr_atm_no_barrier_store(&lock->initiating_exec_ctx_or_null,
+                             (gpr_atm)exec_ctx);
     // first element on this list: add it to the list of combiner locks
     // executing within this exec_ctx
     push_last_on_exec_ctx(exec_ctx, lock);
+  } else {
+    // there may be a race with setting here: if that happens, we may delay
+    // offload for one or two actions, and that's fine
+    gpr_atm initiator =
+        gpr_atm_no_barrier_load(&lock->initiating_exec_ctx_or_null);
+    if (initiator != 0 && initiator != (gpr_atm)exec_ctx) {
+      gpr_atm_no_barrier_store(&lock->initiating_exec_ctx_or_null, 0);
+    }
   }
+  GPR_ASSERT(last & STATE_UNORPHANED);  // ensure lock has not been destroyed
+  assert(cl->cb);
+  cl->error_data.error = error;
+  gpr_mpscq_push(&lock->queue, &cl->next_data.atm_next);
   GPR_TIMER_END("combiner.execute", 0);
 }