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: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

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++) {