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    SubjectRe: Q: perf_install_in_context/perf_event_enable are racy?
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    On Fri, 2011-01-21 at 14:03 +0100, Ingo Molnar wrote:
    > > > But, it seems, there is another problem. Forget about the exiting,
    > > > I can't understand why we can trust current in the code above.
    > > > With __ARCH_WANT_INTERRUPTS_ON_CTXSW schedule() does:
    > > >
    > > > // sets cpuctx->task_ctx = NULL
    > > > perf_event_task_sched_out();
    > > >
    > > > // enables irqs
    > > > prepare_lock_switch();
    > > >
    > > >
    > > > // updates current_task
    > > > switch_to();
    > > >
    > > > What if IPI comes in the window before switch_to() ?
    > > >
    > > > (the same questions for __perf_event_enable).
    > >
    > > Ingo, do you have any insights in that, I think you wrote all that
    > > initially?
    >
    > Not sure. Can an IPI come there - we have irqs disabled usually, dont we?

    Ah, I think I see how that works:

    __perf_event_task_sched_out()
    perf_event_context_sched_out()
    if (do_switch)
    cpuctx->task_ctx = NULL;

    vs

    __perf_install_in_context()
    if (cpu_ctx->task_ctx != ctx)




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