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    SubjectRe: [patch] Re: spin_unlock optimization(i386)
    Gerard wrote:
    > Even if the simple 'mov' may ensure other processors to have a
    > consistent view of the spinlock, it does not prevent the CPU that
    > unlocks from playing with speculative execution around the 'unlock'
    > and perform speculative reads for example. Without a minimal
    > serialization this stuff does not seem safe to me, or at least not for
    > ever.

    If local speculative reads are a problem, perhaps this would be faster
    than "lock; btrl $0,%0":

    "movl $0,%0" then rmb().

    The rmb() does a locked operation but no cross-CPU traffic on a PPro.
    Thus local reads and writes are fully serialised, and bus traffic is
    limited to the write which we believe is read correctly by other
    processors.

    The rmb() can be a separate asm so that GCC can place some register
    operations in between if it's that clever (which it isn't, but it might
    be one day).

    OTOH, I don't see how speculative reads can be a problem. You're
    speculatively reading from /outside/ a locked region so anything goes.

    -- Jamie

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