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    SubjectRe: SMP syncronization on AMD processors (broken?)
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    On Thu, 2005-10-06 at 17:32 +0400, Andrey Savochkin wrote:
    > On Thu, Oct 06, 2005 at 03:19:07PM +0200, Arjan van de Ven wrote:
    > > On Thu, 2005-10-06 at 17:05 +0400, Kirill Korotaev wrote:
    > > > Hello Linus, Andrew and others,
    > > >
    > > > Please help with a not simple question about spin_lock/spin_unlock on
    > > > SMP archs. The question is whether concurrent spin_lock()'s should
    > > > acquire it in more or less "fair" fashinon or one of CPUs can starve any
    > > > arbitrary time while others do reacquire it in a loop.
    > >
    > > spinlocks are designed to not be fair. or rather are allowed to not be.
    > > If you want them to be fair on x86 you need at minimum to put a
    > > cpu_relax() in your busy loop...
    >
    > The question was raised exactly because cpu_relax() doesn't help on these AMD
    > CPUs.
    >
    > Some Pentiums do more than expected from them, and the programs works in a
    > very fair manner even without cpu_relax(), so the question boils down to
    > whether there are some new AMD rules how to write such loops, is it a defect
    > of the CPU, or if we are missing something else.

    the rule basically is "don't write such a loop" though; this is only the
    beginning; because if you have two such things on separate cores of a
    dual core cpu you for sure starve anything outside of that core just the
    same. Eg it goes one level up as well.

    There is no spin_lock_yield() currently and until there is this is just
    a code pattern you should avoid.


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