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>
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@ -9394,6 +9394,11 @@ static void account_event(struct perf_event *event)
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inc = true;
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if (inc) {
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/*
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* We need the mutex here because static_branch_enable()
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* must complete *before* the perf_sched_count increment
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* becomes visible.
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*/
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if (atomic_inc_not_zero(&perf_sched_count))
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goto enabled;
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