only oversubscribe if no pending thread creates

Author: dgrove-oss

src/event/workqueue.c

@@ -207,8 +207,12 @@ _dispatch_workq_monitor_pools(void *context DISPATCH_UNUSED)
 			int32_t limit =
 					mon->num_runnable == 0 ? limit_b : MIN(limit_a, limit_b);
 			if (count + limit > 0) {
-				_dispatch_debug("workq: adding one worker to %s", dq->dq_label);
-				_dispatch_pthread_root_queue_oversubscribe(dq, 1);
+				bool r = _dispatch_pthread_root_queue_oversubscribe(dq, 1);
+				if (r) {
+					_dispatch_debug("workq: requested overscribe worker for %s", dq->dq_label);
+				} else {
+					_dispatch_debug("workq: still pending worker create on %s", dq->dq_label);
+				}
 			} else {
 				_dispatch_debug("workq: %s already over by %d; taking no action",
 						dq->dq_label, -count);

src/queue.c

@@ -3975,16 +3975,22 @@ _dispatch_pthread_root_queue_spawn_workers(dispatch_queue_t dq,
 	} while (--n);
 }
 
-void
+bool
 _dispatch_pthread_root_queue_oversubscribe(dispatch_queue_t dq, int n)
 {
 	dispatch_root_queue_context_t qc = dq->do_ctxt;
 	dispatch_pthread_root_queue_context_t pqc = qc->dgq_ctxt;
 	pthread_attr_t *attr = &pqc->dpq_thread_attr;
 
+	if (!os_atomic_cmpxchg2o(qc, dgq_pending, 0, n, relaxed)) {
+		return false;
+	}
+
 	(void)os_atomic_sub2o(qc, dgq_thread_pool_size, n, release);
 
 	_dispatch_pthread_root_queue_spawn_workers(dq, attr, n);
+
+	return true;
 }
 #endif
 

src/queue_internal.h

@@ -594,7 +594,7 @@ void _dispatch_mgr_queue_drain(void);
 #if DISPATCH_USE_MGR_THREAD && DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES
 void _dispatch_mgr_priority_init(void);
 int32_t _dispatch_pthread_root_queue_thread_pool_size(dispatch_queue_t dq);
-void _dispatch_pthread_root_queue_oversubscribe(dispatch_queue_t dq, int n);
+bool _dispatch_pthread_root_queue_oversubscribe(dispatch_queue_t dq, int n);
 #else
 static inline void _dispatch_mgr_priority_init(void) {}
 #endif