Commit f8933667 authored by Ming Lei's avatar Ming Lei Committed by Jens Axboe
Browse files

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: default avatarChristoph Hellwig <hch@lst.de>
Tested-by: default avatarJarod Wilson <jarod@redhat.com>
Acked-by: default avatarJarod Wilson <jarod@redhat.com>
Signed-off-by: default avatarMing Lei <ming.lei@canonical.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent b816d45f

drivers/block/loop.c

+12 −9
Original line number Diff line number Diff line
@@ -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

+1 −1
Original line number Diff line number Diff line
@@ -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;