linux-kernel - Re: [PATCH] perf: Explain perf_sched

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Message-ID: <87h8wqxyft.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Tue, 29 Aug 2017 17:04:22 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Explain perf_sched_mutex

Alexander Shishkin <alexander.shishkin@...ux.intel.com> writes:

> 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.

"with a short explanation", sigh

>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.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