Re: [PATCH] generic/492: bypass the blkid cache when probing the label

["Darrick J. Wong" <djwong@kernel.org>]

On Sun, Sep 20, 2026 at 05:41:37PM +0800, Baokun Li wrote:
> blkid may serve a stale label from its cache instead of probing the
> device, so the label just set via xfs_io can be missed and the output
> mismatch is non-deterministic.
> 
> Pass "-c /dev/null" so that blkid always probes the on-disk superblock
> directly.  Unlike the low-level probing mode (-p), which requires
> util-linux 2.16 or later, this is also supported by the legacy blkid
> shipped with e2fsprogs.
> 
> Fixes: c3c9630968a6 ("generic: test online label ioctl")
> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>

Seems fine to me; I guess blkid could be racing with whatever updates
the cache...udev?

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  tests/generic/492 | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/tests/generic/492 b/tests/generic/492
> index 51a46b510453..0b4f69b80f17 100755
> --- a/tests/generic/492
> +++ b/tests/generic/492
> @@ -28,14 +28,15 @@ $XFS_IO_PROG -c "label -c" $SCRATCH_MNT
>  $XFS_IO_PROG -c "label" $SCRATCH_MNT
>  
>  # And that userspace can see it now, while mounted
> -# NB: some blkid has trailing whitespace, filter it out here
> +# NB: bypass the blkid cache which may hold a stale label, and filter
> +# out the trailing whitespace some blkid versions emit
>  $XFS_IO_PROG -c "label -s label.$seq" $SCRATCH_MNT
>  $XFS_IO_PROG -c "label" $SCRATCH_MNT
> -blkid -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
> +blkid -c /dev/null -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
>  
>  # And that the it is still there when it's unmounted
>  _scratch_unmount
> -blkid -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
> +blkid -c /dev/null -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
>  
>  # And that it persists after a remount
>  _scratch_mount
> -- 
> 2.43.7
> 
>