On Mon, Aug 29, 2016 at 03:02:13PM +0200, Peter Zijlstra wrote:
> 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);
couldn't we just call smp_call_function_single(cpu_to_read, __perf_event_read,
...
once we read oncpu != -1 ?
__perf_event_read then checks safely if event is active
jirka
---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index eed96b85503f..b99b791c16bb 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3421,6 +3421,7 @@ out:
struct perf_read_data {
struct perf_event *event;
bool group;
+ bool inactive;
int ret;
};
@@ -3562,22 +3563,15 @@ 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;
-
- /*
- * If event is enabled and currently active on a CPU, update the
- * value in the event structure:
- */
- if (event->state == PERF_EVENT_STATE_ACTIVE) {
+ local_cpu = get_cpu();
+ cpu_to_read = find_cpu_to_read(event, local_cpu);
+ if (cpu_to_read != -1) {
struct perf_read_data data = {
.event = event,
.group = group,
.ret = 0,
};
- local_cpu = get_cpu();
- cpu_to_read = find_cpu_to_read(event, local_cpu);
- put_cpu();
-
ret = smp_call_function_single(cpu_to_read, __perf_event_read,
&data, 1);
/* The event must have been read from an online CPU: */
WARN_ON_ONCE(ret);
@@ -3603,6 +3597,7 @@ static int perf_event_read(struct perf_event *event, bool
group)
raw_spin_unlock_irqrestore(&ctx->lock, flags);
}
+ put_cpu();
return ret;
}
