[tip:perfcounters/core] perf_counter: add comment to barrier

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

Commit-ID:  ffd25c2b454557dd5a09b23d2446d1800d497bde
Gitweb:     http://git.kernel.org/tip/ffd25c2b454557dd5a09b23d2446d1800d497bde
Author:     Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Fri, 13 Mar 2009 12:21:30 +0100
Commit:     Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 13 Mar 2009 14:02:43 +0100

perf_counter: add comment to barrier

We need to ensure the enabled=0 write happens before we
start disabling the actual counters, so that a pcm_amd_enable()
will not enable one underneath us.

I think the race is impossible anyway, we always balance the
ops within any one context and perform enable() with IRQs disabled.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090313112301.384074618@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
 arch/x86/kernel/cpu/perf_counter.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_counter.c b/arch/x86/kernel/cpu/perf_counter.c
index 1cedc34..a2e3b76 100644
--- a/arch/x86/kernel/cpu/perf_counter.c
+++ b/arch/x86/kernel/cpu/perf_counter.c
@@ -247,6 +247,10 @@ static u64 pmc_amd_save_disable_all(void)
 
 	enabled = cpuc->enabled;
 	cpuc->enabled = 0;
+	/*
+	 * ensure we write the disable before we start disabling the
+	 * counters proper, so that pcm_amd_enable() does the right thing.
+	 */
 	barrier();
 
 	for (idx = 0; idx < nr_counters_generic; idx++) {