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    SubjectRe: Q: perf_install_in_context/perf_event_enable are racy?
    On 01/27, Peter Zijlstra wrote:
    > Right, so the fact of introducing extra scheduling makes me feel
    > uncomfortable... the whole purpose is to observe without perturbing (as
    > much as possible).

    Yes, agreed.

    Well, otoh the patch removes the code which sets ->task_ctx from
    __perf_install_in_context() and __perf_event_enable(), and perhaps
    we could simplify the things further, but anyway I agree.

    > Should I think suffice to get the ctx in sync with the task state, we've
    > got the following cases:
    > 1) task is in the middle of scheduling in
    > 2) task is in the middle of scheduling out
    > 3) task is running
    > Without __ARCH_WANT_INTERRUPT_ON_CTXSW everything is boring and works,
    > 1: the IPI will be delayed until 3, 2: the IPI finds another task and
    > the next schedule in will sort things.
    > With, however, things are more interesting. 2 seems to be adequately
    > covered by the patch I just send, the IPI will bail and the next
    > sched-in of the relevant task will pick matters up. 1 otoh doesn't seem
    > covered, the IPI will bail, leaving us stranded.

    Hmm, yes... Strangely, I missed that when I was thinking about in_ctxsw.

    Perhaps, we can change task_oncpu_function_call() so that it returns
    -EAGAIN in case it hits in_ctxsw != 0? If the caller sees -EAGAIN, it
    should always retry even if !ctx->is_active.


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