workqueue: Fix spurious sanity check failures in destroy_workqueue()

Before actually destrying a workqueue, destroy_workqueue() checks
whether it's actually idle.  If it isn't, it prints out a bunch of
warning messages and leaves the workqueue dangling.  It unfortunately
has a couple issues.

* Mayday list queueing increments pwq's refcnts which gets detected as
  busy and fails the sanity checks.  However, because mayday list
  queueing is asynchronous, this condition can happen without any
  actual work items left in the workqueue.

* Sanity check failure leaves the sysfs interface behind too which can
  lead to init failure of newer instances of the workqueue.

This patch fixes the above two by

* If a workqueue has a rescuer, disable and kill the rescuer before
  sanity checks.  Disabling and killing is guaranteed to flush the
  existing mayday list.

* Remove sysfs interface before sanity checks.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Marcin Pawlowski <mpawlowski@fb.com>
Reported-by: "Williams, Gerald S" <gerald.s.williams@intel.com>
Cc: stable@vger.kernel.org

kernel/workqueue.c

@@ -4325,9 +4325,28 @@ void destroy_workqueue(struct workqueue_struct *wq)
 	struct pool_workqueue *pwq;
 	int node;
 
+	/*
+	 * Remove it from sysfs first so that sanity check failure doesn't
+	 * lead to sysfs name conflicts.
+	 */
+	workqueue_sysfs_unregister(wq);
+
 	/* drain it before proceeding with destruction */
 	drain_workqueue(wq);
 
+	/* kill rescuer, if sanity checks fail, leave it w/o rescuer */
+	if (wq->rescuer) {
+		struct worker *rescuer = wq->rescuer;
+
+		/* this prevents new queueing */
+		spin_lock_irq(&wq_mayday_lock);
+		wq->rescuer = NULL;
+		spin_unlock_irq(&wq_mayday_lock);
+
+		/* rescuer will empty maydays list before exiting */
+		kthread_stop(rescuer->task);
+	}
+
 	/* sanity checks */
 	mutex_lock(&wq->mutex);
 	for_each_pwq(pwq, wq) {
@@ -4359,11 +4378,6 @@ void destroy_workqueue(struct workqueue_struct *wq)
 	list_del_rcu(&wq->list);
 	mutex_unlock(&wq_pool_mutex);
 
-	workqueue_sysfs_unregister(wq);
-
-	if (wq->rescuer)
-		kthread_stop(wq->rescuer->task);
-
 	if (!(wq->flags & WQ_UNBOUND)) {
 		wq_unregister_lockdep(wq);
 		/*