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    SubjectRe: [PATCH] perf_counter: Check task on counter read IPI
    On Fri, 2009-08-14 at 15:39 +1000, Paul Mackerras wrote:
    > In general, code in perf_counter.c that is called through an IPI
    > checks, for per-task counters, that the counter's task is still the
    > current task. This is to handle the race condition where the cpu
    > switches from the task we want to another task in the interval between
    > sending the IPI and the IPI arriving and being handled on the target
    > CPU.
    > For some reason, __perf_counter_read is missing this check, yet there
    > is no reason why the race condition can't occur. This adds a check
    > that the current task is the one we want. If it isn't, we just
    > return. In that case the counter->count value should be up to date,
    > since it will have been updated when the counter was scheduled out,
    > which must have happened since the IPI was sent.
    > Signed-off-by: Paul Mackerras <>
    > ---
    > I don't have an example of an actual failure due to this race, but it
    > seems obvious that it could occur and we need to guard against it, so
    > I think this should go in .31.

    Hmm, right.

    However those other sites have retry loops in the caller, but callers of
    __perf_counter_read() do not. Granted, I'm not sure what they should
    retry on exactly, but this patch trades an invalid update to a missing

    While I think the balance tips towards favouring the missing update, its
    not really much of an improvement.

    I guess we could keep a sequence count with the update and loop until it
    gets increased or something?

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