Re: [PATCH 11/12] xfs: drop s_umount over opening the log and RT devices

Darrick J. Wong
2023-08-02T16:32:24Z

On Wed, Aug 02, 2023 at 05:41:30PM +0200, Christoph Hellwig wrote:
> Just like get_tree_bdev needs to drop s_umount when opening the main
> device, we need to do the same for the xfs log and RT devices to avoid a
> potential lock order reversal with s_unmount for the mark_dead path.
>
> It might be preferable to just drop s_umount over ->fill_super entirely,
> but that will require a fairly massive audit first, so we'll do the easy
> version here first.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/xfs/xfs_super.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 8185102431301d..d5042419ed9997 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -448,17 +448,21 @@ STATIC int
> xfs_open_devices(
> struct xfs_mount *mp)
> {
> - struct block_device *ddev = mp->m_super->s_bdev;
> + struct super_block *sb = mp->m_super;
> + struct block_device *ddev = sb->s_bdev;
> struct block_device *logdev = NULL, *rtdev = NULL;
> int error;
>
> + /* see get_tree_bdev why this is needed and safe */

Which part of get_tree_bdev? Is it this?

/*
* s_umount nests inside open_mutex during
* __invalidate_device(). blkdev_put() acquires
* open_mutex and can't be called under s_umount. Drop
* s_umount temporarily. This is safe as we're
* holding an active reference.
*/
up_write(&s->s_umount);
blkdev_put(bdev, fc->fs_type);
down_write(&s->s_umount);

<confused>

> + up_write(&sb->s_umount);
> +
> /*
> * Open real time and log devices - order is important.
> */
> if (mp->m_logname) {
> error = xfs_blkdev_get(mp, mp->m_logname, &logdev);
> if (error)
> - return error;
> + goto out_unlock;
> }
>
> if (mp->m_rtname) {
> @@ -496,7 +500,10 @@ xfs_open_devices(
> mp->m_logdev_targp = mp->m_ddev_targp;
> }
>
> - return 0;
> + error = 0;
> +out_unlock:
> + down_write(&sb->s_umount);

Isn't down_write taking s_umount? I think the label should be
out_relock or something less misleading.

--D

> + return error;
>
> out_free_rtdev_targ:
> if (mp->m_rtdev_targp)
> @@ -508,7 +515,7 @@ xfs_open_devices(
> out_close_logdev:
> if (logdev && logdev != ddev)
> xfs_blkdev_put(mp, logdev);
> - return error;
> + goto out_unlock;
> }
>
> /*
> --
> 2.39.2
>