workqueue: Don't rely on wq->rescuer to stop rescuer

The commit1 def98c8 ("workqueue: Fix spurious sanity check failures
in destroy_workqueue()") tries to fix spurious sanity check failures by
stopping send_mayday() via setting wq->rescuer to NULL.

But it fails to stop the pwq->mayday_node requeuing in the rescuer, and
the commit2 e66b39a ("workqueue: Fix pwq ref leak in
rescuer_thread()") fixes it by checking wq->rescuer which is the result
of commit1.

Both commits together really fix spurious sanity check failures caused
by the rescuer, but they both use a convoluted method by relying on
wq->rescuer state rather than the real count of work items.

Actually __WQ_DESTROYING and drain_workqueue() together already stop
send_mayday() by draining all the work items and ensuring no new work
item requeuing.

And the more proper fix to stop the pwq->mayday_node requeuing in the
rescuer is from commit3 4f3f4cf ("workqueue: avoid unneeded
requeuing the pwq in rescuer thread") and renders the checking of
wq->rescuer in commit2 unnecessary.

So __WQ_DESTROYING, drain_workqueue() and commit3 together fix spurious
sanity check failures introduced by the rescuer.

Just remove the convoluted code of using wq->rescuer.

Signed-off-by: Lai Jiangshan <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Lai Jiangshan

kernel/workqueue.c

@@ -3539,10 +3539,9 @@ static int rescuer_thread(void *__rescuer)
 			if (pwq->nr_active && need_to_create_worker(pool)) {
 				raw_spin_lock(&wq_mayday_lock);
 				/*
-				 * Queue iff we aren't racing destruction
-				 * and somebody else hasn't queued it already.
+				 * Queue iff somebody else hasn't queued it already.
 				 */
-				if (wq->rescuer && list_empty(&pwq->mayday_node)) {
+				if (list_empty(&pwq->mayday_node)) {
 					get_pwq(pwq);
 					list_add_tail(&pwq->mayday_node, &wq->maydays);
 				}
@@ -5905,16 +5904,10 @@ void destroy_workqueue(struct workqueue_struct *wq)
 
 	/* kill rescuer, if sanity checks fail, leave it w/o rescuer */
 	if (wq->rescuer) {
-		struct worker *rescuer = wq->rescuer;
-
-		/* this prevents new queueing */
-		raw_spin_lock_irq(&wq_mayday_lock);
-		wq->rescuer = NULL;
-		raw_spin_unlock_irq(&wq_mayday_lock);
-
 		/* rescuer will empty maydays list before exiting */
-		kthread_stop(rescuer->task);
-		kfree(rescuer);
+		kthread_stop(wq->rescuer->task);
+		kfree(wq->rescuer);
+		wq->rescuer = NULL;
 	}
 
 	/*