loop: don't destroy lo->workqueue in __loop_clr_fd

There is no need to destroy the workqueue when clearing unbinding
a loop device from a backing file.  Not doing so on the other hand
avoid creating a complex lock dependency chain involving the global
system_transition_mutex.

Based on a patch from Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>.

Reported-by: syzbot+6479585dfd4dedd3f7e1@syzkaller.appspotmail.com
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: syzbot+6479585dfd4dedd3f7e1@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/20220330052917.2566582-16-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -812,7 +812,6 @@ struct loop_worker {
 };
 
 static void loop_workfn(struct work_struct *work);
-static void loop_rootcg_workfn(struct work_struct *work);
 
 #ifdef CONFIG_BLK_CGROUP
 static inline int queue_on_root_worker(struct cgroup_subsys_state *css)
@@ -1050,20 +1049,19 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
 	    !file->f_op->write_iter)
 		lo->lo_flags |= LO_FLAGS_READ_ONLY;
 
+	if (!lo->workqueue) {
 		lo->workqueue = alloc_workqueue("loop%d",
 						WQ_UNBOUND | WQ_FREEZABLE,
-					0,
-					lo->lo_number);
+						0, lo->lo_number);
 		if (!lo->workqueue) {
 			error = -ENOMEM;
 			goto out_unlock;
 		}
+	}
 
 	disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE);
 	set_disk_ro(lo->lo_disk, (lo->lo_flags & LO_FLAGS_READ_ONLY) != 0);
 
-	INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
-	INIT_LIST_HEAD(&lo->rootcg_cmd_list);
 	lo->use_dio = lo->lo_flags & LO_FLAGS_DIRECT_IO;
 	lo->lo_device = bdev;
 	lo->lo_backing_file = file;
@@ -1143,10 +1141,6 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
 	if (!release)
 		blk_mq_freeze_queue(lo->lo_queue);
 
-	destroy_workqueue(lo->workqueue);
-	loop_free_idle_workers(lo, true);
-	del_timer_sync(&lo->timer);
-
 	spin_lock_irq(&lo->lo_lock);
 	filp = lo->lo_backing_file;
 	lo->lo_backing_file = NULL;
@@ -1750,6 +1744,10 @@ static void lo_free_disk(struct gendisk *disk)
 {
 	struct loop_device *lo = disk->private_data;
 
+	if (lo->workqueue)
+		destroy_workqueue(lo->workqueue);
+	loop_free_idle_workers(lo, true);
+	del_timer_sync(&lo->timer);
 	mutex_destroy(&lo->lo_mutex);
 	kfree(lo);
 }
@@ -2013,6 +2011,8 @@ static int loop_add(int i)
 	lo->lo_number		= i;
 	spin_lock_init(&lo->lo_lock);
 	spin_lock_init(&lo->lo_work_lock);
+	INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
+	INIT_LIST_HEAD(&lo->rootcg_cmd_list);
 	disk->major		= LOOP_MAJOR;
 	disk->first_minor	= i << part_shift;
 	disk->minors		= 1 << part_shift;