On 05/06/2021 15:09, Christoph Hellwig wrote:
> Commit c76f48eb5c08 ("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@xxxxxxxxxx>.
>
> Reported-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Fixes: c76f48eb5c08 ("block: take bd_mutex around delete_partitions in del_gendisk")
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> drivers/block/loop.c | 25 +++++++------------------
> drivers/block/loop.h | 1 +
> 2 files changed, 8 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index d58d68f3c7cd..76e12f3482a9 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1879,29 +1879,18 @@ 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)
> @@ -2285,7 +2274,7 @@ 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);
> diff --git a/drivers/block/loop.h b/drivers/block/loop.h
> index a3c04f310672..5beb959b94d3 100644
> --- a/drivers/block/loop.h
> +++ b/drivers/block/loop.h
> @@ -22,6 +22,7 @@ enum {
> Lo_unbound,
> Lo_bound,
> Lo_rundown,
> + Lo_deleting,
> };
>
> struct loop_func_table;
>
Tested and no longer hangs with the stress-ng loop test. Thank you for
the speedy turnaround on the fix.
Tested-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Colin
