The event->hw.last_period is possible to zero, thus it will
cause divide_by_zero later in perf_swevent_set_period().

This patch checks event->hw.last_period before invoke
perf_swevent_set_period() and replaces "event->hw" with "hwc".

Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
---
 kernel/perf_event.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 403d180..ccba741 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -4050,8 +4050,8 @@ static void perf_swevent_overflow(struct perf_event *event, u64 overflow,
 	struct hw_perf_event *hwc = &event->hw;
 	int throttle = 0;
 
-	data->period = event->hw.last_period;
-	if (!overflow)
+	data->period = hwc->last_period;
+	if (!overflow && hwc->last_period)
 		overflow = perf_swevent_set_period(event);
 
 	if (hwc->interrupts == MAX_INTERRUPTS)
-- 
1.6.0.4

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