From 5bce9db1894c998c5b85a34036d679ea6517668f Mon Sep 17 00:00:00 2001 From: Alexander Shishkin Date: Tue, 29 Aug 2017 17:01:03 +0300 Subject: perf/core: Explain perf_sched_mutex To clarify why atomic_inc_return(&perf_sched_events) is not sufficient and a mutex is needed to order static branch enabling vs the atomic counter increment, this adds a comment with a short explanation. Signed-off-by: Alexander Shishkin Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20170829140103.6563-1-alexander.shishkin@linux.intel.com Signed-off-by: Ingo Molnar --- kernel/events/core.c | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'kernel/events') diff --git a/kernel/events/core.c b/kernel/events/core.c index 6bc21e202ae4..5ee62714f9a6 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -9394,6 +9394,11 @@ static void account_event(struct perf_event *event) inc = true; if (inc) { + /* + * We need the mutex here because static_branch_enable() + * must complete *before* the perf_sched_count increment + * becomes visible. + */ if (atomic_inc_not_zero(&perf_sched_count)) goto enabled; -- cgit