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block: loop: fix deadlock between open and remove

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Commit c76f48eb5c ("block: take bd_mutex around delete_partitions in
del_gendisk") adds disk->part0->bd_mutex in del_gendisk(), this way
causes the following AB/BA deadlock between removing loop and opening
loop:

 1) loop_control_ioctl(LOOP_CTL_REMOVE)
     -> mutex_lock(&loop_ctl_mutex)
     -> del_gendisk
         -> mutex_lock(&disk->part0->bd_mutex)

 2) blkdev_get_by_dev
     -> mutex_lock(&disk->part0->bd_mutex)
     -> lo_open
         -> mutex_lock(&loop_ctl_mutex)

Add a new Lo_deleting state to remove the need for clearing
->private_data and thus holding loop_ctl_mutex in the ioctl
LOOP_CTL_REMOVE path.

Based on an analysis and earlier patch from
Ming Lei <ming.lei@redhat.com>.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: c76f48eb5c ("block: take bd_mutex around delete_partitions in del_gendisk")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210605140950.5800-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Christoph Hellwig 2021-06-05 17:09:50 +03:00 committed by Jens Axboe
parent 41fe8d088e
commit 990e78116d
2 changed files with 8 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
drivers/block/loop.c
View File

@ -1878,29 +1878,18 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
static int lo_open(struct block_device *bdev, fmode_t mode)
{
struct loop_device *lo;
struct loop_device *lo = bdev->bd_disk->private_data;
int err;
/*
* take loop_ctl_mutex to protect lo pointer from race with
* loop_control_ioctl(LOOP_CTL_REMOVE), however, to reduce contention
* release it prior to updating lo->lo_refcnt.
*/
err = mutex_lock_killable(&loop_ctl_mutex);
if (err)
return err;
lo = bdev->bd_disk->private_data;
if (!lo) {
mutex_unlock(&loop_ctl_mutex);
return -ENXIO;
}
err = mutex_lock_killable(&lo->lo_mutex);
mutex_unlock(&loop_ctl_mutex);
if (err)
return err;
atomic_inc(&lo->lo_refcnt);
if (lo->lo_state == Lo_deleting)
err = -ENXIO;
else
atomic_inc(&lo->lo_refcnt);
mutex_unlock(&lo->lo_mutex);
return 0;
return err;
}
static void lo_release(struct gendisk *disk, fmode_t mode)
@ -2284,7 +2273,7 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd,
mutex_unlock(&lo->lo_mutex);
break;
}
lo->lo_disk->private_data = NULL;
lo->lo_state = Lo_deleting;
mutex_unlock(&lo->lo_mutex);
idr_remove(&loop_index_idr, lo->lo_number);
loop_remove(lo);

1
drivers/block/loop.h
View File

@ -22,6 +22,7 @@ enum {
Lo_unbound,
Lo_bound,
Lo_rundown,
Lo_deleting,
};
struct loop_func_table;