Commit f6ab91ad authored Jun 04, 2010 by Peter Zijlstra Committed by Ingo Molnar Jun 08, 2010

perf: Fix signed comparison in perf_adjust_period()



Frederic reported that frequency driven swevents didn't work properly
and even caused a division-by-zero error.

It turns out there are two bugs, the division-by-zero comes from a
failure to deal with that in perf_calculate_period().

The other was more interesting and turned out to be a wrong comparison
in perf_adjust_period(). The comparison was between an s64 and u64 and
got implicitly converted to an unsigned comparison. The problem is
that period_left is typically < 0, so it ended up being always true.

Cure this by making the local period variables s64.

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 58cc1a9e

kernel/perf_event.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -1507,6 +1507,9 @@ do { \
		divisor = nsec * frequency;
		}

		if (!divisor)
		return dividend;

		return div64_u64(dividend, divisor);
		}

		@@ -1529,7 +1532,7 @@ static int perf_event_start(struct perf_event *event)
		static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
		{
		struct hw_perf_event *hwc = &event->hw;
		u64 period, sample_period;
		s64 period, sample_period;
		s64 delta;

		period = perf_calculate_period(event, nsec, count);

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