[PATCH tip/rt/head] perf: Mark software events as irqsafe

[Arnaldo Carvalho de Melo <acme@infradead.org>]

From: Peter Zijlstra <peterz@infradead.org>

Noticed while using 'perf top' on a machine without a supported PMU on
2.6.33.7-rt29, when top falls back to using the "cpu-clock" software
event.

Reproduced on other machines where the problem was masked due to the
existence and use of hardware events, by explicitely asking for a
software event using:

perf top -e cpu-clock

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@infradead.org>

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 6dfaf5b..b9fbc17 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -4238,6 +4238,7 @@ static void perf_swevent_start_hrtimer(struct perf_event *event)
 	struct hw_perf_event *hwc = &event->hw;
 
 	hrtimer_init(&hwc->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hwc->hrtimer.irqsafe = 1;
 	hwc->hrtimer.function = perf_swevent_hrtimer;
 	if (hwc->sample_period) {
 		u64 period;