Re: [RFC PATCH] libfrog: make cmn_err() emit each message atomically to avoid torn output

On Wed, Jul 15, 2026 at 07:28:21PM +0200, Avinesh Kumar wrote:
> From: Avinesh Kumar <[email protected]>
> 
> fstests xfs/033 fails sporadically with a spurious blank line in the
> xfs_repair output:
> 
> - output mismatch (see /opt/xfstests/results//xfs/033.out.bad)
>     --- tests/xfs/033.out	2026-06-24 15:52:51.000000000 -0400
>     +++ /opt/xfstests/results//xfs/033.out.bad	2026-07-14 18:54:46.582495041 -0400
>     @@ -103,6 +103,7 @@
>      Phase 3 - for each AG...
>              - scan and clear agi unlinked lists...
>              - process known inodes and perform inode discovery...
>     +
>      bad magic number 0xffff on inode INO
>      bad version number 0xffffffff on inode INO
>      inode identifier 18446744073709551615 mismatch on inode INO
> 
> Root cause is in cmn_err() (libfrog/util.c), which emits a message
> and its newline as two separate writes to unbuffered stderr.
> xfs_repair's threads all share stderr. If one is preempted between the
> two writes, another thread's line lands in between:
> 
> (snips from `cat -A 033.raw`) -
> Phase 3 - for each AG...$
>         - scan and clear agi unlinked lists...$
>         - process known inodes and perform inode discovery...$
> Metadata corruption detected at 0x445dd3, xfs_inode block 0x80/0x4000        - agno = 0$
> $
> bad CRC for inode 128$
> bad magic number 0x0 on inode 128$
> 
> which should be like -
> 
> Phase 3 - for each AG...$
>         - scan and clear agi unlinked lists...$
>         - process known inodes and perform inode discovery...$
> Metadata corruption detected at 0x445dd3, xfs_inode block 0x80/0x4000$
>         - agno = 0$
> bad CRC for inode 130$
> bad magic number 0x0 on inode 130$
> 
> _filter_repair() strips the noise line it glued onto but not the lone
> newline, which then fails the golden diff.
> 
> Make the two writes atomic.
> 
> Signed-off-by: Avinesh Kumar <[email protected]>
> ---
> 
> Hi, please give feedback if this should be fixed in a different way.
> Thanks!
> 
>  libfrog/util.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/libfrog/util.c b/libfrog/util.c
> index 5bae5bab..3b6df917 100644
> --- a/libfrog/util.c
> +++ b/libfrog/util.c
> @@ -116,8 +116,10 @@ cmn_err(int level, char *fmt, ...)
>  	va_list	ap;
>  
>  	va_start(ap, fmt);
> +	flockfile(stderr);
>  	vfprintf(stderr, fmt, ap);
>  	fputs("\n", stderr);
> +	funlockfile(stderr);

I think do_error() in xfs_repair.c is going to need the same treatment
but otherwise this chunk looks ok to me.

--D

>  	va_end(ap);
>  }
>  
> -- 
> 2.55.0
> 
>