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    SubjectRe: [PATCH] splice : two smp_mb() can be omitted
    On Tue, Oct 31 2006, Nick Piggin wrote:
    > Eric Dumazet wrote:
    > >This patch deletes two calls to smp_mb() that were done after
    > >mutex_unlock() that contains an implicit memory barrier.
    >
    > Uh, there is nothing that says mutex_unlock or any unlock
    > functions contain an implicit smp_mb(). What is given is that the
    > lock and unlock obey aquire and release memory ordering,
    > respectively.
    >
    > a = x;
    > xxx_unlock
    > b = y;
    >
    > In this situation, the load of y can be executed before that of x.
    > And some architectures will even do so (i386 can, because the
    > unlock is an unprefixed store; ia64 can, because it uses a release
    > barrier in the unlock).
    >
    > Whenever you rely on orderings of things *outside* locks (even
    > partially outside), you do need to be very careful about barriers
    > and can't rely on locks to do the right thing for you.

    Good point, we should not make any assumptions on the way the
    architecture implements the mutexes.

    --
    Jens Axboe

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