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    SubjectRe: [PATCH 0/3] coupled cpuidle state support
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    On Wed, Dec 21, 2011 at 1:44 AM, Arjan van de Ven <arjan@linux.intel.com> wrote:
    > On 12/21/2011 10:40 AM, Colin Cross wrote:
    >
    >>> this smells fundamentally racey to me; you can get an interrupt one
    >>> cycle after you think you're done, but before the last guy enters WFI...
    >>>
    >>> how do you solve that issue ?
    >>
    >> All the cpus have interrupts off when they increment the counter, so
    >> they cannot receive an interrupt.  If an interrupt is pending on one
    >> of those cpus, it will be handled later when WFI aborts due to the
    >> pending interrupt.
    >
    > ... but this leads to cases where you're aborting before other cpus are
    > entering..... so your "last guy in" doesn't really work, since while cpu
    > 0 thinks it's the last guy, cpu 1 is already on the way out/out
    > already...  (heck it might already be going back to sleep if your idle
    > code can run fast, like in the size of a cache miss)

    Once a cpu has incremented the counter, it has no way out unless either
    1: another cpu (that hasn't incremented the counter yet) receives an
    interrupt, aborts idle, and clears its idle flag
    or
    2: all cpus enter the ready counter, and call the cpuidle driver's
    enter function.

    In your example, cpu 1 has incremented the counter, so it cannot be on
    the way out unless cpu 0 aborts (in which case it will not increment
    the counter, and the counter will never be equal to the number of
    cpus), or unless cpu 0 turns off its interrupts and incrementes the
    counter (in which case neither cpu can return until after the cpuidle
    driver's enter function has been called on all cpus).
    --
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