Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch

Hisashi Hifumi
2008-10-07T10:14:31Z

At 15:43 08/10/07, Andi Kleen wrote:
>Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp> writes:
>
>> Hi Andrew.
>>
>> Currently reading or writing file->f_pos is not atomic on 32bit environment,
>> so two or more simultaneous access can corrupt file->f_pos value.
>> There are some past discussions about this issue, but this is not fixed yet.
>> http://marc.info/?l=linux-kernel&m=120764199819899&w=2
>> http://marc.info/?l=linux-kernel&m=114490379102476&w=2
>
>Have you benchmarked if cmpxchg is cheaper than the seqlock? It's not
>clear to me a seqlock is really the right locking primitive for
>this. Normally seqlocks should be used when reading is much more
>frequent than writing, but it's doubtful that this is actually the
>case for f_pos.

Maybe cmpxchg8b is good for i486 or later x86, but i386 or other architectures
that do not have similar instruction needs some locking primitive. I think lazy
seqlock is one option for making file->f_pos access atomic.