[PATCH] perf/x86/rapl: fix deadlock in rapl_pmu_event_stop

[Duoming Zhou]

There is a deadlock in rapl_pmu_event_stop(), the process is
shown below:

    (thread 1)                 |        (thread 2)
rapl_pmu_event_stop()          | rapl_hrtimer_handle()
 ...                           |  if (!pmu->n_active)
 raw_spin_lock_irqsave() //(1) |  ...
  ...                          |
  hrtimer_cancel()             |  raw_spin_lock_irqsave() //(2)
  (block forever)

We hold pmu->lock in position (1) and use hrtimer_cancel() to wait
rapl_hrtimer_handle() to stop, but rapl_hrtimer_handle() also need
pmu->lock in position (2). As a result, the rapl_pmu_event_stop()
will be blocked forever.

This patch extracts hrtimer_cancel() from the protection of
raw_spin_lock_irqsave(). As a result, the rapl_hrtimer_handle() could
obtain the pmu->lock. In order to prevent race conditions, we put
"if (!pmu->n_active)" in rapl_hrtimer_handle() under the protection
of raw_spin_lock_irqsave().

Fixes: 65661f96d3b3 ("perf/x86: Add RAPL hrtimer support")
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
 arch/x86/events/rapl.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c
index 77e3a47af5a..97c71538d01 100644
--- a/arch/x86/events/rapl.c
+++ b/arch/x86/events/rapl.c
@@ -219,11 +219,11 @@ static enum hrtimer_restart rapl_hrtimer_handle(struct hrtimer *hrtimer)
 	struct perf_event *event;
 	unsigned long flags;
 
+	raw_spin_lock_irqsave(&pmu->lock, flags);
+
 	if (!pmu->n_active)
 		return HRTIMER_NORESTART;
 
-	raw_spin_lock_irqsave(&pmu->lock, flags);
-
 	list_for_each_entry(event, &pmu->active_list, active_entry)
 		rapl_event_update(event);
 
@@ -281,8 +281,11 @@ static void rapl_pmu_event_stop(struct perf_event *event, int mode)
 	if (!(hwc->state & PERF_HES_STOPPED)) {
 		WARN_ON_ONCE(pmu->n_active <= 0);
 		pmu->n_active--;
-		if (pmu->n_active == 0)
+		if (!pmu->n_active) {
+			raw_spin_unlock_irqrestore(&pmu->lock, flags);
 			hrtimer_cancel(&pmu->hrtimer);
+			raw_spin_lock_irqsave(&pmu->lock, flags);
+		}
 
 		list_del(&event->active_entry);
 
-- 
2.17.1