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

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

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
> 
>