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 <hch@lst.de>
Tested-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/loop.c

@@ -831,7 +831,7 @@ static int loop_clr_fd(struct loop_device *lo)
 	 * <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;
@@ -840,6 +840,9 @@ static int loop_clr_fd(struct loop_device *lo)
 	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_backing_file = NULL;
@@ -871,6 +874,8 @@ static int loop_clr_fd(struct loop_device *lo)
 	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;
@@ -1330,9 +1335,7 @@ static int lo_open(struct block_device *bdev, fmode_t mode)
 		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;
@@ -1343,11 +1346,10 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
 	struct loop_device *lo = disk->private_data;
 	int err;
 
-	mutex_lock(&lo->lo_ctl_mutex);
-
-	if (--lo->lo_refcnt)
-		goto out;
+	if (atomic_dec_return(&lo->lo_refcnt))
+		return;
 
+	mutex_lock(&lo->lo_ctl_mutex);
 	if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) {
 		/*
 		 * In autoclear mode, stop the loop thread
@@ -1601,6 +1603,7 @@ static int loop_add(struct loop_device **l, int i)
 		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;
@@ -1718,7 +1721,7 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd,
 			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

@@ -28,7 +28,7 @@ struct loop_func_table;
 
 struct loop_device {
 	int		lo_number;
-	int		lo_refcnt;
+	atomic_t	lo_refcnt;
 	loff_t		lo_offset;
 	loff_t		lo_sizelimit;
 	int		lo_flags;