block: loop: don't hold lo_ctl_mutex in lo_open

The lo_ctl_mutex is held for running all ioctl handlers, and
in some ioctl handlers, ioctl_by_bdev(BLKRRPART) is called for
rereading partitions, which requires bd_mutex.

So it is easy to cause failure because trylock(bd_mutex) may
fail inside blkdev_reread_part(), and follows the lock context:

blkid or other application:
->open()
->mutex_lock(bd_mutex)
->lo_open()
->mutex_lock(lo_ctl_mutex)

losetup(set fd ioctl):
->mutex_lock(lo_ctl_mutex)
->ioctl_by_bdev(BLKRRPART)
->trylock(bd_mutex)

This patch trys to eliminate the ABBA lock dependency by removing
lo_ctl_mutext in lo_open() with the following approach:

1) make lo_refcnt as atomic_t and avoid acquiring lo_ctl_mutex in lo_open():
- for open vs. add/del loop, no any problem because of loop_index_mutex
- freeze request queue during clr_fd, so I/O can't come until
clearing fd is completed, like the effect of holding lo_ctl_mutex
in lo_open
- both open() and release() have been serialized by bd_mutex already

2) don't hold lo_ctl_mutex for decreasing/checking lo_refcnt in
lo_release(), then lo_ctl_mutex is only required for the last release.

Reviewed-by: Christoph Hellwig <[e-mail]>
Tested-by: Jarod Wilson <[e-mail]>
Acked-by: Jarod Wilson <[e-mail]>
Signed-off-by: Ming Lei <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/loop.c

···
 	 * <dev>/do something like mkfs/losetup -d <dev> causing the losetup -d
 	 * command to fail with EBUSY.
 	 */
-	if (lo->lo_refcnt > 1) {
+	if (atomic_read(&lo->lo_refcnt) > 1) {
 		lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
 		mutex_unlock(&lo->lo_ctl_mutex);
 		return 0;
···
 
 	if (filp == NULL)
 		return -EINVAL;
+
+	/* freeze request queue during the transition */
+	blk_mq_freeze_queue(lo->lo_queue);
 
 	spin_lock_irq(&lo->lo_lock);
 	lo->lo_state = Lo_rundown;
···
 	lo->lo_state = Lo_unbound;
 	/* This is safe: open() is still holding a reference. */
 	module_put(THIS_MODULE);
+	blk_mq_unfreeze_queue(lo->lo_queue);
+
 	if (lo->lo_flags & LO_FLAGS_PARTSCAN && bdev)
 		ioctl_by_bdev(bdev, BLKRRPART, 0);
 	lo->lo_flags = 0;
···
 		goto out;
 	}
 
-	mutex_lock(&lo->lo_ctl_mutex);
-	lo->lo_refcnt++;
-	mutex_unlock(&lo->lo_ctl_mutex);
+	atomic_inc(&lo->lo_refcnt);
 out:
 	mutex_unlock(&loop_index_mutex);
 	return err;
···
 	struct loop_device *lo = disk->private_data;
 	int err;
 
+	if (atomic_dec_return(&lo->lo_refcnt))
+		return;
+
 	mutex_lock(&lo->lo_ctl_mutex);
-
-	if (--lo->lo_refcnt)
-		goto out;
-
 	if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) {
 		/*
 		 * In autoclear mode, stop the loop thread
···
 		disk->flags |= GENHD_FL_NO_PART_SCAN;
 	disk->flags |= GENHD_FL_EXT_DEVT;
 	mutex_init(&lo->lo_ctl_mutex);
+	atomic_set(&lo->lo_refcnt, 0);
 	lo->lo_number		= i;
 	spin_lock_init(&lo->lo_lock);
 	disk->major		= LOOP_MAJOR;
···
 			mutex_unlock(&lo->lo_ctl_mutex);
 			break;
 		}
-		if (lo->lo_refcnt > 0) {
+		if (atomic_read(&lo->lo_refcnt) > 0) {
 			ret = -EBUSY;
 			mutex_unlock(&lo->lo_ctl_mutex);
 			break;

drivers/block/loop.h

···
 
 struct loop_device {
 	int		lo_number;
-	int		lo_refcnt;
+	atomic_t	lo_refcnt;
 	loff_t		lo_offset;
 	loff_t		lo_sizelimit;
 	int		lo_flags;