use dispatch_unfair_locks around registered worker critical sections

Author: dgrove-oss

src/event/workqueue.c

@@ -51,7 +51,7 @@ static volatile int32_t _dispatch_workq_runnable_workers;
  * The desired number of runnable worker threads
  * for a workqueue (assuming sufficient work).
  */
-static int _dispatch_workq_target_runnable_workers;
+static int32_t _dispatch_workq_target_runnable_workers;
 
 #if DISPATCH_ENABLE_PWQ_KEXT
 /* Are we using user-level or kext based management? */
@@ -63,10 +63,10 @@ static bool _dispatch_workq_kext_active;
  */
 typedef struct dispatch_workq_manager_s {
 	/*
-	 * Tracking of registered workers; all updates and reads are performed
-	 * holding the mutex.
+	 * Tracking of registered workers; all updates and reads
+	 * are performed while holding the lock.
 	 */
-	pthread_mutex_t registered_worker_mutex;
+	dispatch_unfair_lock_s registered_worker_lock;
 	int num_registered_workers;
 	pid_t *registered_workers;
 
@@ -116,17 +116,15 @@ dispatch_workq_worker_register(dispatch_queue_t root_q)
 #endif
 	bool rc;
 	int tid = syscall(SYS_gettid);
-	int r = pthread_mutex_lock(&_dispatch_workq_manager.registered_worker_mutex);
-	(void)dispatch_assume_zero(r);
+	_dispatch_unfair_lock_lock(&_dispatch_workq_manager.registered_worker_lock);
 	if (_dispatch_workq_manager.num_registered_workers < WORKQ_MAX_TRACKED_WORKERS-1) {
 		int worker_id = _dispatch_workq_manager.num_registered_workers++;
 		_dispatch_workq_manager.registered_workers[worker_id] = tid;
 		rc = true;
 	} else {
 		rc = false;
 	}
-	r = pthread_mutex_unlock(&_dispatch_workq_manager.registered_worker_mutex);
-	(void)dispatch_assume_zero(r);
+	_dispatch_unfair_lock_unlock(&_dispatch_workq_manager.registered_worker_lock);
 
 	return rc;
 }
@@ -146,8 +144,7 @@ dispatch_workq_worker_unregister(dispatch_queue_t root_q)
 	}
 #endif
 	int tid = syscall(SYS_gettid);
-	int r = pthread_mutex_lock(&_dispatch_workq_manager.registered_worker_mutex);
-	(void)dispatch_assume_zero(r);
+	_dispatch_unfair_lock_lock(&_dispatch_workq_manager.registered_worker_lock);
 	for (int i=0; i<_dispatch_workq_manager.num_registered_workers; i++) {
 		if (_dispatch_workq_manager.registered_workers[i] == tid) {
 			int last = _dispatch_workq_manager.num_registered_workers - 1;
@@ -157,8 +154,7 @@ dispatch_workq_worker_unregister(dispatch_queue_t root_q)
 			break;
 		}
 	}
-	r = pthread_mutex_unlock(&_dispatch_workq_manager.registered_worker_mutex);
-	(void)dispatch_assume_zero(r);
+	_dispatch_unfair_lock_unlock(&_dispatch_workq_manager.registered_worker_lock);
 }
 
 
@@ -173,26 +169,24 @@ _dispatch_workq_count_runnable_workers(void)
 	char path[128];
 	char buf[4096];
 	int running_count = 0;
-	int r;
 
 	memset(buf, 0, sizeof(buf));
 
-	r = pthread_mutex_lock(&_dispatch_workq_manager.registered_worker_mutex);
-	(void)dispatch_assume_zero(r);
+	_dispatch_unfair_lock_lock(&_dispatch_workq_manager.registered_worker_lock);
 
 	for (int i=0; i<_dispatch_workq_manager.num_registered_workers; i++) {
 		pid_t worker_pid = _dispatch_workq_manager.registered_workers[i];
 		int fd;
 		size_t bytes_read = -1;
 
-		r = snprintf(path, sizeof(path), "/proc/%d/stat", worker_pid);
+		int r = snprintf(path, sizeof(path), "/proc/%d/stat", worker_pid);
 		(void)dispatch_assume(r > 0 && r < sizeof(path));
 
 		fd = open(path, O_RDONLY | O_NONBLOCK);
 		if (unlikely(fd == -1)) {
 			// Unable to open file.
 			// Must mean worker exited uncleanly (without executing _dispatch_worker_unregister())
-			// Clean up by removing pid and decrementing number of workers in pool
+			// Clean up by removing pid and decrementing number of registered workers
 			_dispatch_debug("workq: Unable to open /proc/%d/stat; removing worker from monitoring list", worker_pid);
 			int last = _dispatch_workq_manager.num_registered_workers-1;
 			_dispatch_workq_manager.registered_workers[i] = _dispatch_workq_manager.registered_workers[last];
@@ -219,8 +213,7 @@ _dispatch_workq_count_runnable_workers(void)
 		}
 	}
 
-	r = pthread_mutex_unlock(&_dispatch_workq_manager.registered_worker_mutex);
-	(void)dispatch_assume_zero(r);
+	_dispatch_unfair_lock_unlock(&_dispatch_workq_manager.registered_worker_lock);
 
 	return running_count;
 }
@@ -264,11 +257,6 @@ _dispatch_workq_init_once(void *context DISPATCH_UNUSED)
 		return;
 	}
 #endif
-	int r;
-
-	r = pthread_mutex_init(&_dispatch_workq_manager.registered_worker_mutex, NULL);
-	(void)dispatch_assume_zero(r);
-
 	_dispatch_workq_manager.registered_workers =
 			_dispatch_calloc(WORKQ_MAX_TRACKED_WORKERS, sizeof(pid_t));