[PATCH] perf: Explain perf_sched_mutex

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

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 an short explanation.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
---
 kernel/events/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index efe09b8c38..2c8719b635 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9569,6 +9569,11 @@ 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;
 
-- 
2.14.1