workqueue: fix locking in retry path of maybe_create_worker()

maybe_create_worker() mismanaged locking when worker creation fails
and it has to retry.  Fix locking and simplify lock manipulation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Yong Zhang <yong.zhang@windriver.com>

kernel/workqueue.c

@@ -1442,14 +1442,14 @@ static bool maybe_create_worker(struct global_cwq *gcwq)
 	if (!need_to_create_worker(gcwq))
 		return false;
 restart:
+	spin_unlock_irq(&gcwq->lock);
+
 	/* if we don't make progress in MAYDAY_INITIAL_TIMEOUT, call for help */
 	mod_timer(&gcwq->mayday_timer, jiffies + MAYDAY_INITIAL_TIMEOUT);
 
 	while (true) {
 		struct worker *worker;
 
-		spin_unlock_irq(&gcwq->lock);
-
 		worker = create_worker(gcwq, true);
 		if (worker) {
 			del_timer_sync(&gcwq->mayday_timer);
@@ -1462,15 +1462,13 @@ static bool maybe_create_worker(struct global_cwq *gcwq)
 		if (!need_to_create_worker(gcwq))
 			break;
 
-		spin_unlock_irq(&gcwq->lock);
 		__set_current_state(TASK_INTERRUPTIBLE);
 		schedule_timeout(CREATE_COOLDOWN);
-		spin_lock_irq(&gcwq->lock);
+
 		if (!need_to_create_worker(gcwq))
 			break;
 	}
 
-	spin_unlock_irq(&gcwq->lock);
 	del_timer_sync(&gcwq->mayday_timer);
 	spin_lock_irq(&gcwq->lock);
 	if (need_to_create_worker(gcwq))