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    SubjectRe: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch
    On Tuesday 07 October 2008 21:29, Andi Kleen wrote:
    > > 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
    > We have a cmpxchg emulation on 386. That works because only UP 386s are
    > supported, so it can be done in software.
    > > seqlock is one option for making file->f_pos access atomic.
    > The question is if it's the right option. At least all the common
    > operations on fds (read/write) are all writers, not readers.

    Common operations are read, do something, write. So seqlocks then cost
    one atomic operation, a couple of memory barriers (all noops on x86),
    and some predictable branches etc.

    cmpxchg based would require 2 lock ; cmpxchg8b on 32-bit. Fairly heavy.
    Also I don't think we have generic accessors to do this, so I think
    that is for another project.

    Anyway, I think importantly this creates some usable accessors for the
    f_pos problem. I think we actually need to touch a _lot_ of code to
    cover all f_pos accesses in the kernel, but I guess this gets the ball

    So.. is everyone agreed that corrupting f_pos is a bad thing? (serious
    question) If so, then we should get something like this merged sooner
    rather than later.

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      Last update: 2008-10-07 18:31    [W:0.022 / U:25.936 seconds]
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