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    SubjectRe: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

    On Fri, 7 Nov 2008, David Howells wrote:

    > Steven Rostedt <rostedt@goodmis.org> wrote:
    >
    > > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could
    > > > be required so it works also on UP systems safely wrt interrupts).
    > >
    > > smp_rmb turns into a compiler barrier on UP and should prevent the below
    > > description.
    >
    > Note that that does not guarantee that the two reads will be done in the order
    > you want. The compiler barrier _only_ affects the compiler. It does not stop
    > the CPU from doing the reads in any order it wants. You need something
    > stronger than smp_rmb() if you need the reads to be so ordered.

    For reading hardware devices that can indeed be correct. But for normal
    memory access on a uniprocessor, if the CPU were to reorder the reads that
    would effect the actual algorithm then that CPU is broken.

    read a
    <--- interrupt - should see read a here before read b is done.
    read b

    Now the fact that one of the reads is a hardware clock, then this
    statement might not be too strong. But the fact that it is a clock, and
    not some memory mapped device register, I still think smp_rmb is
    sufficient.

    -- Steve



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