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From:   Alexander Shishkin <alexander.shishkin@linux.intel.com>
To:     Yabin Cui <yabinc@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Yabin Cui <yabinc@google.com>,
        alexander.shishkin@linux.intel.com
Subject: Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head.
In-Reply-To: <20190515003059.23920-1-yabinc@google.com>
References: <20190515003059.23920-1-yabinc@google.com>
Date:   Wed, 15 May 2019 09:51:07 +0300
Message-ID: <87ef50xlb8.fsf@ashishki-desk.ger.corp.intel.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Yabin Cui <yabinc@google.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