[tip:perf/core] perf: Fix signed comparison in perf_adjust_period()

From: tip-bot for Peter Zijlstra
Date: Tue Jun 08 2010 - 16:55:49 EST

Commit-ID: f6ab91add6355e231e1c47897027b2a6ee4fa268
Gitweb: http://git.kernel.org/tip/f6ab91add6355e231e1c47897027b2a6ee4fa268
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
AuthorDate: Fri, 4 Jun 2010 15:18:01 +0200
Committer: Ingo Molnar <mingo@xxxxxxx>
CommitDate: Tue, 8 Jun 2010 18:43:00 +0200

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@xxxxxxxxx>
Tested-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
kernel/perf_event.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 31d6afe..ff86c55 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -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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