workqueue: Fix pwq ref leak in rescuer_thread()

008847f ("workqueue: allow rescuer thread to do more work.") made
the rescuer worker requeue the pwq immediately if there may be more
work items which need rescuing instead of waiting for the next mayday
timer expiration.  Unfortunately, it doesn't check whether the pwq is
already on the mayday list and unconditionally gets the ref and moves
it onto the list.  This doesn't corrupt the list but creates an
additional reference to the pwq.  It got queued twice but will only be
removed once.

This leak later can trigger pwq refcnt warning on workqueue
destruction and prevent freeing of the workqueue.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Williams, Gerald S" <gerald.s.williams@intel.com>
Cc: NeilBrown <neilb@suse.de>
Cc: stable@vger.kernel.org # v3.19+

kernel/workqueue.c

@@ -2533,8 +2533,14 @@ static int rescuer_thread(void *__rescuer)
 			 */
 			if (need_to_create_worker(pool)) {
 				spin_lock(&wq_mayday_lock);
-				get_pwq(pwq);
-				list_move_tail(&pwq->mayday_node, &wq->maydays);
+				/*
+				 * Queue iff we aren't racing destruction
+				 * and somebody else hasn't queued it already.
+				 */
+				if (wq->rescuer && list_empty(&pwq->mayday_node)) {
+					get_pwq(pwq);
+					list_add_tail(&pwq->mayday_node, &wq->maydays);
+				}
 				spin_unlock(&wq_mayday_lock);
 			}
 		}
@@ -4374,8 +4380,8 @@ void destroy_workqueue(struct workqueue_struct *wq)
 	for_each_pwq(pwq, wq) {
 		spin_lock_irq(&pwq->pool->lock);
 		if (WARN_ON(pwq_busy(pwq))) {
-			pr_warning("%s: %s has the following busy pwq (refcnt=%d)\n",
-				   __func__, wq->name, pwq->refcnt);
+			pr_warning("%s: %s has the following busy pwq\n",
+				   __func__, wq->name);
 			show_pwq(pwq);
 			spin_unlock_irq(&pwq->pool->lock);
 			mutex_unlock(&wq->mutex);
@@ -4670,7 +4676,8 @@ static void show_pwq(struct pool_workqueue *pwq)
 	pr_info("  pwq %d:", pool->id);
 	pr_cont_pool_info(pool);
 
-	pr_cont(" active=%d/%d%s\n", pwq->nr_active, pwq->max_active,
+	pr_cont(" active=%d/%d refcnt=%d%s\n",
+		pwq->nr_active, pwq->max_active, pwq->refcnt,
 		!list_empty(&pwq->mayday_node) ? " MAYDAY" : "");
 
 	hash_for_each(pool->busy_hash, bkt, worker, hentry) {