[tip:perfcounters/core] perf_counter, x86: protect per-cpu variables with compile barriers only

tip-bot for Robert Richter
2009-04-29T13:06:13Z

Commit-ID: 527e26af3741a2168986d8b82653ffe173891324
Gitweb: http://git.kernel.org/tip/527e26af3741a2168986d8b82653ffe173891324
Author: Robert Richter <robert.richter@amd.com>
AuthorDate: Wed, 29 Apr 2009 12:47:02 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 29 Apr 2009 14:51:02 +0200

perf_counter, x86: protect per-cpu variables with compile barriers only

Per-cpu variables needn't to be protected with cpu barriers
(smp_wmb()). Protection is only needed for preemption on the same cpu
(rescheduling or the nmi handler). This can be done using a compiler
barrier only.

[ Impact: micro-optimization ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1241002046-8832-6-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

---
arch/x86/kernel/cpu/perf_counter.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_counter.c b/arch/x86/kernel/cpu/perf_counter.c
index 75a0903..ad663d5 100644
--- a/arch/x86/kernel/cpu/perf_counter.c
+++ b/arch/x86/kernel/cpu/perf_counter.c
@@ -673,7 +673,7 @@ try_generic:
/*
* Make it visible before enabling the hw:
*/
- smp_wmb();
+ barrier();

__hw_perf_counter_set_period(counter, hwc, idx);
__pmc_generic_enable(counter, hwc, idx);
@@ -745,7 +745,7 @@ static void pmc_generic_disable(struct perf_counter *counter)
* Make sure the cleared pointer becomes visible before we
* (potentially) free the counter:
*/
- smp_wmb();
+ barrier();

/*
* Drain the remaining delta count out of a counter