[PATCH v2 4/8] md: defer the io_opt update out of the sync thread

@@ -671,6 +671,7 @@ void mddev_put(struct mddev *mddev)
 
 static void md_safemode_timeout(struct timer_list *t);
 static void md_start_sync(struct work_struct *ws);
+static void md_io_opt_work(struct work_struct *ws);
 
 static void active_io_release(struct percpu_ref *ref)
 {

@@ -794,6 +795,7 @@ int mddev_init(struct mddev *mddev)
 	mddev->level = LEVEL_NONE;
 
 	INIT_WORK(&mddev->sync_work, md_start_sync);
+	INIT_WORK(&mddev->io_opt_work, md_io_opt_work);
 	INIT_WORK(&mddev->del_work, mddev_delayed_delete);
 
 	return 0;

@@ -6330,20 +6332,51 @@ int mddev_stack_rdev_into(struct mddev *mddev, struct md_rdev *rdev,
 EXPORT_SYMBOL_GPL(mddev_stack_rdev_into);
 
 /* update the optimal I/O size after a reshape */
-void mddev_update_io_opt(struct mddev *mddev, unsigned int nr_stripes)
+static void md_io_opt_work(struct work_struct *ws)
 {
+	struct mddev *mddev = container_of(ws, struct mddev, io_opt_work);
+	struct request_queue *q = mddev->gendisk->queue;
 	struct queue_limits lim;
 
+	/*
+	 * Nothing is held here, so take q->limits_lock in the order the rest
+	 * of md uses: before the suspend, see md_start_sync().
+	 */
+	lim = queue_limits_start_update(q);
+	if (mddev_suspend(mddev, false) < 0) {
+		queue_limits_cancel_update(q);
+		return;
+	}
+	lim.io_opt = lim.io_min * READ_ONCE(mddev->io_opt_nr_stripes);
+	if (queue_limits_commit_update(q, &lim))
+		pr_err("%s: could not apply queue limits\n", mdname(mddev));
+	mddev_resume(mddev);
+}
+
+void mddev_update_io_opt(struct mddev *mddev, unsigned int nr_stripes,
+			 struct queue_limits *lim)
+{
 	if (mddev_is_dm(mddev))
 		return;
 
-	/* don't bother updating io_opt if we can't suspend the array */
-	if (mddev_suspend(mddev, false) < 0)
+	/*
+	 * With an update owned by the caller just change it in place; it is
+	 * committed, and the array resumed, by whoever started it.  Taking
+	 * q->limits_lock here would nest it inside reconfig_mutex and the
+	 * suspend, which deadlocks, see md_start_sync().
+	 */
+	if (lim) {
+		lim->io_opt = lim->io_min * nr_stripes;
 		return;
-	lim = queue_limits_start_update(mddev->gendisk->queue);
-	lim.io_opt = lim.io_min * nr_stripes;
-	queue_limits_commit_update(mddev->gendisk->queue, &lim);
-	mddev_resume(mddev);
+	}
+
+	/*
+	 * Called from the sync thread, which md_reap_sync_thread() waits for
+	 * with reconfig_mutex held, so q->limits_lock cannot be taken here
+	 * either.  Hand it to a work item that holds neither.
+	 */
+	WRITE_ONCE(mddev->io_opt_nr_stripes, nr_stripes);
+	queue_work(md_misc_wq, &mddev->io_opt_work);
 }
 EXPORT_SYMBOL_GPL(mddev_update_io_opt);
 

@@ -7139,6 +7172,8 @@ static void __md_stop(struct mddev *mddev)
 {
 	struct md_personality *pers = mddev->pers;
 
+	/* the deferred io_opt update suspends the array, so let it finish */
+	flush_work(&mddev->io_opt_work);
 	mddev_detach(mddev);
 	md_bitmap_destroy(mddev);
 	spin_lock(&mddev->lock);

@@ -554,6 +554,10 @@ struct mddev {
 	/* used for register new sync thread */
 	struct work_struct sync_work;
 
+	/* deferred io_opt update, see mddev_update_io_opt() */
+	struct work_struct	io_opt_work;
+	unsigned int		io_opt_nr_stripes;
+
 	/* "lock" protects:
 	 *   flush_bio transition from NULL to !NULL
 	 *   rdev superblocks, events

@@ -1056,7 +1060,8 @@ int mddev_stack_rdev_into(struct mddev *mddev, struct md_rdev *rdev,
  * is added with the array's current limits.
  */
 #define MDDEV_STACK_SKIP	((struct queue_limits *)ERR_PTR(-EAGAIN))
-void mddev_update_io_opt(struct mddev *mddev, unsigned int nr_stripes);
+void mddev_update_io_opt(struct mddev *mddev, unsigned int nr_stripes,
+			 struct queue_limits *lim);
 
 extern const struct block_device_operations md_fops;
 

@@ -4932,7 +4932,7 @@ static void end_reshape(struct r10conf *conf)
 	conf->reshape_safe = MaxSector;
 	spin_unlock_irq(&conf->device_lock);
 
-	mddev_update_io_opt(conf->mddev, raid10_nr_stripes(conf));
+	mddev_update_io_opt(conf->mddev, raid10_nr_stripes(conf), NULL);
 	conf->fullsync = 0;
 }
 

@@ -8800,7 +8800,7 @@ static void end_reshape(struct r5conf *conf)
 		wake_up(&conf->wait_for_reshape);
 
 		mddev_update_io_opt(conf->mddev,
-			conf->raid_disks - conf->max_degraded);
+			conf->raid_disks - conf->max_degraded, NULL);
 	}
 }