[PATCH] perf_counter: fix possible /0 bug in swcounters

[Peter Zijlstra <peterz@infradead.org>]

We have a race in the swcounter stuff where we can start counting a
counter that has never been enabled, this leads to a /0 situation.

The below avoids the /0 but doesn't close the race, this would need a
new counter state.

The race is due to perf_swcounter_is_counting() which cannot discern
between disabled due to scheduled out, and disabled for any other
reason.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/perf_counter.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 53abcbe..2b90d26 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -4103,6 +4103,7 @@ perf_counter_alloc(struct perf_counter_attr *attr,
 	hwc->sample_period = attr->sample_period;
 	if (attr->freq && attr->sample_freq)
 		hwc->sample_period = 1;
+	hwc->last_period = hwc->sample_period;
 
 	atomic64_set(&hwc->period_left, hwc->sample_period);