Re: [PATCH] fstests: fix bug with _check_dmesg "some pattern"

Darrick J. Wong
2025-11-21T19:21:26Z

On Fri, Nov 21, 2025 at 12:55:34PM -0500, Josef Bacik wrote:
> I've been rigging up Claude to run fstests via an MCP, and part of this
> means it mounts a 9p file system to save the fstests results. This was
> giving me occasional ENODATA errors, which I dug into. Turns out it's a
> race between truncate and read on 9p which is completely normal for 9p,
> but it's uncovering an actual bug in _check_dmesg. The way bash works is
> when you do
>
> cat foo | whatever > foo
>
> bash will setup the various communication channels first, which is the |
> and >, and then it will execute the commands. This means that it
> truncates foo FIRST, and then it launches cat. Which means we'd race
> with truncate and sometimes get data, sometimes get nothing. And even
> worse on 9p we'd get ENODATA because we get a 0 read back when we
> thought the isize was reasonable.
>
> Fix this by changing this case in _check_dmesg to redirect to another
> file, and move that file over $seqres.dmesg to be checked later.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> common/rc | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/common/rc b/common/rc
> index a10ac177..947685df 100644
> --- a/common/rc
> +++ b/common/rc
> @@ -4844,7 +4844,14 @@ _check_dmesg()
> # test failures. Custom filters are then applied afterwards.
> _dmesg_since_test_start | _check_dmesg_filter > $seqres.dmesg
> if [ -n "$1" ]; then
> - cat $seqres.dmesg | $1 > $seqres.dmesg
> + # We use $seqres.dmesg.tmp because shell will setup the
> + # redirections first, which means we would truncate
> + # $seqres.dmesg before cat'ing it. This is racy so sometimes it
> + # would work, but othertimes it would silently fail, or in the
> + # case of 9p you'd get ENODATA. We must redirect into a
> + # temporary file and then move it.
> + cat $seqres.dmesg | $1 > $seqres.dmesg.tmp && \
> + mv $seqres.dmesg.tmp $seqres.dmesg

I wonder if this would be simpler put:

if [ -n "$1" ]; then
_dmesg_since_test_start | _check_dmesg_filter | $1 > $seqres.dme
else
_dmesg_since_test_start | _check_dmesg_filter > $seqres.dmesg
fi

Fewer tempfiles, etc.

--D

> fi
>
> grep -E -q -e "kernel BUG at" \
> --
> 2.51.1
>
>