linux-kernel - Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data

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Message-ID: <87ef50xlb8.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Wed, 15 May 2019 09:51:07 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Yabin Cui <yabinc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Yabin Cui <yabinc@...gle.com>,
        alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head.

Yabin Cui <yabinc@...gle.com> writes:

> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 674b35383491..0b9aefe13b04 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -54,8 +54,10 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
>  	 * IRQ/NMI can happen here, which means we can miss a head update.
>  	 */
>  
> -	if (!local_dec_and_test(&rb->nest))
> +	if (local_read(&rb->nest) > 1) {
> +		local_dec(&rb->nest);

What stops rb->nest changing between local_read() and local_dec()?

Regards,
--
Alex