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>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/arch/x86/kernel/cpu/perf_counter.c b/arch/x86/kernel/cpu/perf_counter.c
index 1cedc3468ce5b85f1a91db1938631f882c4de64c..a2e3b76bfdc1504985d216ca8d01b6b1a58d0018 100644 (file)
--- 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++) {