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    SubjectRe: Q: perf_install_in_context/perf_event_enable are racy?

    * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

    > On Thu, 2011-01-20 at 20:30 +0100, Oleg Nesterov wrote:
    > > On 01/19, Oleg Nesterov wrote:
    > > >
    > > > Also. I believe there are more problems in perf_install_event(), but
    > > > I need to recheck.
    > >
    > > Help! I can't believe it can be so trivially wrong, but otoh I can't
    > > understand how this can be correct.
    > >
    > > So, ignoring details and !task case, __perf_install_in_context() does:
    > >
    > > if (cpuctx->task_ctx || ctx->task != current)
    > > return;
    > >
    > > cpuctx->task_ctx = ctx;
    > > event_sched_in(event);
    > >
    > > Stupid question, what if this task has already passed
    > > perf_event_exit_task() and thus it doesn't have ->perf_event_ctxp[] ?
    > > Given that perf_event_context_sched_out() does nothing if !ctx, who
    > > will event_sched_out() this event?
    > >
    > > OK, even if I am right this is trivial, we just need the additional
    > > check.
    >
    > Indeed (or do the cleanup from put_ctx(), but that's too complex a
    > change I think).
    >
    > > 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?

    Thanks,

    Ingo


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