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

[Avinesh Kumar <avinesh.kumar@suse.com>]

Hello Darrick,


On 7/15/26 9:09 PM, Darrick J. Wong wrote:
> On Wed, Jul 15, 2026 at 07:28:21PM +0200, Avinesh Kumar wrote:
>> From: Avinesh Kumar <avinesh.kumar@suse.com>
>>
>> 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 <avinesh.kumar@suse.com>
>> ---
>>
>> 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.
> 
Thanks for the review. I can send a follow up patch for do_error(),
or do you want these to go in the same patch?

Regards,
Avinesh

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