On Mon, Aug 29, 2016 at 12:03:09PM +0200, Peter Zijlstra wrote:
> @@ -1802,8 +1802,18 @@ event_sched_out(struct perf_event *event,
>
> event->tstamp_stopped = tstamp;
> event->pmu->del(event, 0);
> - event->oncpu = -1;
> - event->state = PERF_EVENT_STATE_INACTIVE;
> +
> + WRITE_ONCE(event->state, PERF_EVENT_STATE_INACTIVE);
> + /*
> + * pmu::del() will have updated the event count. Now mark it inactive,
> + * but take care to clear ->oncpu after the INACTIVE store, such that
> + * while ->state == ACTIVE, ->oncpu must be valid.
> + *
> + * See event_sched_in(), perf_event_restart() and perf_event_read().
> + */
> + smp_wmb();
> + WRITE_ONCE(event->oncpu, -1);
> +
> if (event->pending_disable) {
> event->pending_disable = 0;
> event->state = PERF_EVENT_STATE_OFF;
> @@ -2015,8 +2025,10 @@ event_sched_in(struct perf_event *event,
>
> WRITE_ONCE(event->oncpu, smp_processor_id());
> /*
> - * Order event::oncpu write to happen before the ACTIVE state
> - * is visible.
> + * Order event::oncpu write to happen before the ACTIVE state is
> + * visible, such that when we observe ACTIVE, oncpu must be correct.
> + *
> + * Matches the smp_rmb() in perf_event_restart().
> */
> smp_wmb();
> WRITE_ONCE(event->state, PERF_EVENT_STATE_ACTIVE);
Urgh.. that cannot work either, because now perf_event_read() can race
against event_sched_in(). Since that's no longer crossed.
> @@ -3561,28 +3576,36 @@ u64 perf_event_read_local(struct perf_event *event)
>
> static int perf_event_read(struct perf_event *event, bool group)
> {
> - int ret = 0, cpu_to_read, local_cpu;
> + int ret = 0, cpu_to_read, local_cpu, state;
> +
> + local_cpu = get_cpu(); /* disable preemption to hold off hotplut */
> + cpu_to_read = READ_ONCE(event->oncpu);
> + /*
> + * Matches smp_wmb() from event_sched_out(), ->oncpu must be valid
> + * IFF we observe ACTIVE.
> + */
> + smp_rmb();
> + state = READ_ONCE(event->state);
The best I can come up with is something like:
do {
state = READ_ONCE(event->state);
if (state != ACTIVE)
break;
smp_rmb();
cpu = READ_ONCE(event->cpu);
smp_rmb();
} while (READ_ONCE(event->state) != state);
And I suppose perf_event_restart() should do the same thing... Let me
ponder this a wee bit more.
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