[RFC][PATCH 7/7] perf: Cure event->pending_disable race

From: Peter Zijlstra
Date: Fri Feb 19 2016 - 09:50:21 EST


Because event_sched_out() checks event->pending_disable _before_
actually disabling the event, it can happen that the event fires after
it checks but before it gets disabled.

This would leave event->pending_disable set and the queued irq_work
will try and process it.

However, if the event trigger was during schedule(), the event might
have been de-scheduled by the time the irq_work runs, and
perf_event_disable_local() will fail.

Fix this by checking event->pending_disable _after_ we call
event->pmu->del(). This depends on the latter being a compiler
barrier, such that the compiler does not lift the load and re-creates
the problem.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
kernel/events/core.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1696,14 +1696,14 @@ event_sched_out(struct perf_event *event

perf_pmu_disable(event->pmu);

+ event->tstamp_stopped = tstamp;
+ event->pmu->del(event, 0);
+ event->oncpu = -1;
event->state = PERF_EVENT_STATE_INACTIVE;
if (event->pending_disable) {
event->pending_disable = 0;
event->state = PERF_EVENT_STATE_OFF;
}
- event->tstamp_stopped = tstamp;
- event->pmu->del(event, 0);
- event->oncpu = -1;

if (!is_software_event(event))
cpuctx->active_oncpu--;