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    SubjectRE: [PATCH 4/5] kgdb: Use atomic operators which use barriers

    On Thu, 8 Apr 2010, Will Deacon wrote:
    > I simply used smp_mb() as a way to solve this ARM-specific problem. I think
    > Russell objects to this largely because this problem affects a particular
    > scenario of busy-wait loops and changing the definition of cpu_relax() adds
    > barriers to code that doesn't necessarily require them.

    How expensive is a smp_mb() on arm?

    And by "expensive" I don't mean so much performance of the instruction
    itself (after all, we _are_ just busy-looping), but more about things like
    power and perhaps secondary effects (does it cause memory traffic, for

    Also, I have to say that _usually_ the problem with non-timely cache
    updates in not on the reading side, but on the writing side - ie the other
    CPU may be buffering writes indefinitely and the writes will go out only
    as a response to bus cycles or the write buffers filling up. In which case
    the reader can't really do much about it.

    But your comment for the "smp_mb()" patch seems to imply that it's
    literally a matter of cache access priorities:

    "On the ARM11MPCore processor [where loads are prioritised over stores],
    spinning in such a loop will prevent the write buffer from draining."

    and in that case I would say that the correct thing _definitely_ is to
    make sure that the loop simply is never so tight that. Maybe you can do
    that without an smp_mb(), by just making whatever "cpu_relax()" does slow
    enough (something that stalls the pipeline or whatever?)

    But if smp_mb() is cheap, then that sounds like the right solution.


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