Re: [tip:perf/core] perf/core: Check return value of the perf_event_read() IPI

[Jiri Olsa <jolsa@redhat.com>]

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;
 }