[tip:perf/urgent] perf_event: Cleanup for cpu_clock_perf_event_update()

[tip-bot for Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

Commit-ID:  ec89a06fd4e12301f11ab039ee07d2353a18addc
Gitweb:     http://git.kernel.org/tip/ec89a06fd4e12301f11ab039ee07d2353a18addc
Author:     Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
AuthorDate: Wed, 9 Dec 2009 11:30:36 +0800
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 9 Dec 2009 09:56:27 +0100

perf_event: Cleanup for cpu_clock_perf_event_update()

Using atomic64_xchg() instead of atomic64_read() and
atomic64_set().

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <4B1F19DC.90204@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/perf_event.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 77641ae..94e1b28 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -4079,8 +4079,7 @@ static void cpu_clock_perf_event_update(struct perf_event *event)
 	u64 now;
 
 	now = cpu_clock(cpu);
-	prev = atomic64_read(&event->hw.prev_count);
-	atomic64_set(&event->hw.prev_count, now);
+	prev = atomic64_xchg(&event->hw.prev_count, now);
 	atomic64_add(now - prev, &event->count);
 }