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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 <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170829140103.6563-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Alexander Shishkin 2017-08-29 17:01:03 +03:00 committed by Ingo Molnar
parent 4c4de7d3c8
commit 5bce9db189
1 changed files with 5 additions and 0 deletions
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5
kernel/events/core.c
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@ -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;