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    SubjectRe: [PATCH,RFC] perf: panic due to inclied cpu context task_ctx value
    On 03/26, Peter Zijlstra wrote:
    >
    > On Sat, 2011-03-26 at 18:35 +0100, Oleg Nesterov wrote:
    > > On 03/26, Oleg Nesterov wrote:
    > > >
    > > > On 03/26, Peter Zijlstra wrote:
    > > > >
    > > > > diff --git a/kernel/perf_event.c b/kernel/perf_event.c
    > > > > index c75925c..e9e4e35 100644
    > > > > --- a/kernel/perf_event.c
    > > > > +++ b/kernel/perf_event.c
    > > > > @@ -1073,6 +1073,8 @@ event_sched_out(struct perf_event *event,
    > > > > if (!is_software_event(event))
    > > > > cpuctx->active_oncpu--;
    > > > > ctx->nr_active--;
    > > > > + if (!ctx->nr_active && cpuctx->task_ctx == ctx)
    > > > > + cpuctx->task_ctx = NULL;
    > > >
    > > > If we clear cpuctx->task_ctx, we should also clear ctx->is_active.
    >
    > Right.

    Wait... Yes, we have to clear ctx->is_active, otherwise we break, say,
    perf_install_in_context().

    But if we clear ->is_active we break perf_event_enable(). Suppose we
    are doing ioctl(PERF_EVENT_IOC_DISABLE) + ioctl(PERF_EVENT_IOC_ENABLE).
    PERF_EVENT_IOC_DISABLE can sched_out the last event, but _IOC_ENABLE
    treats ctx->is_active == F as "it is not running".


    Btw, why ctx_sched_out() checks nr_events under perf_pmu_disable() ?

    Oleg.



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