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    SubjectRe: [PATCH 6/24] make atomic_read() behave consistently on frv
    On Wed, Aug 15, 2007 at 10:13:49PM +0200, Segher Boessenkool wrote:
    > >>>>>Well if there is only one memory location involved, then smp_rmb()
    > >>>>>isn't
    > >>>>>going to really do anything anyway, so it would be incorrect to use
    > >>>>>it.
    > >>>>
    > >>>>rmb() orders *any* two reads; that includes two reads from the same
    > >>>>location.
    > >>>
    > >>>If the two reads are to the same location, all CPUs I am aware of
    > >>>will maintain the ordering without need for a memory barrier.
    > >>
    > >>That's true of course, although there is no real guarantee for that.
    > >
    > >A CPU that did not provide this property ("cache coherence") would be
    > >most emphatically reviled.
    > That doesn't have anything to do with coherency as far as I can see.
    > It's just about the order in which a CPU (speculatively) performs the
    > loads
    > (which isn't necessarily the same as the order in which it executes the
    > corresponding instructions, even).

    Please check the definition of "cache coherence".

    Summary: the CPU is indeed within its rights to execute loads and stores
    to a single variable out of order, -but- only if it gets the same result
    that it would have obtained by executing them in order. Which means that
    any reordering of accesses by a single CPU to a single variable will be
    invisible to the software.

    > >So we are pretty safe assuming that CPUs
    > >will provide it.
    > Yeah, pretty safe. I just don't like undocumented assumptions :-)

    Can't help you there! ;-)

    Thanx, Paul
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      Last update: 2007-08-15 22:43    [W:0.021 / U:6.040 seconds]
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