workqueue: note the nested NOT_RUNNING test in worker_clr_flags() isn't a noop

The nested NOT_RUNNING test in worker_clr_flags() is slightly
misleading in that if NOT_RUNNING were a single flag the nested test
would be always %true and thus noop.  Add a comment noting that the
test isn't a noop.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Hillf Danton <dhillf@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

kernel/workqueue.c

@@ -768,7 +768,11 @@ static inline void worker_clr_flags(struct worker *worker, unsigned int flags)
 
 	worker->flags &= ~flags;
 
-	/* if transitioning out of NOT_RUNNING, increment nr_running */
+	/*
+	 * If transitioning out of NOT_RUNNING, increment nr_running.  Note
+	 * that the nested NOT_RUNNING is not a noop.  NOT_RUNNING is mask
+	 * of multiple flags, not a single flag.
+	 */
 	if ((flags & WORKER_NOT_RUNNING) && (oflags & WORKER_NOT_RUNNING))
 		if (!(worker->flags & WORKER_NOT_RUNNING))
 			atomic_inc(get_gcwq_nr_running(gcwq->cpu));