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

    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.



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