workqueue: move rescuer pool detachment to the end

In 51697d39 ("workqueue: use generic attach/detach routine for
rescuers"), The rescuer detaches itself from the pool before put_pwq()
so that the put_unbound_pool() will not destroy the rescuer-attached
pool.

It is unnecessary.  worker_detach_from_pool() can be used as the last
statement to access to the pool just like the regular workers,
put_unbound_pool() will wait for it to detach and then free the pool.

So we move the worker_detach_from_pool() down, make it coincide with
the regular workers.

tj: Minor description update.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -2253,15 +2253,10 @@ static int rescuer_thread(void *__rescuer)
 				move_linked_works(work, scheduled, &n);
 
 		process_scheduled_works(rescuer);
-		spin_unlock_irq(&pool->lock);
-
-		worker_detach_from_pool(rescuer, pool);
-
-		spin_lock_irq(&pool->lock);
 
 		/*
 		 * Put the reference grabbed by send_mayday().  @pool won't
-		 * go away while we're holding its lock.
+		 * go away while we're still attached to it.
 		 */
 		put_pwq(pwq);
 
@@ -2274,8 +2269,11 @@ static int rescuer_thread(void *__rescuer)
 			wake_up_worker(pool);
 
 		rescuer->pool = NULL;
-		spin_unlock(&pool->lock);
-		spin_lock(&wq_mayday_lock);
+		spin_unlock_irq(&pool->lock);
+
+		worker_detach_from_pool(rescuer, pool);
+
+		spin_lock_irq(&wq_mayday_lock);
 	}
 
 	spin_unlock_irq(&wq_mayday_lock);