Re: [PATCH v4 02/11] coresight: tmc-etr: Add barrier after updating AUX ring buffer

From: Suzuki K Poulose <suzuki.poulose@arm.com>
To: Leo Yan <leo.yan@linaro.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Mike Leach <mike.leach@linaro.org>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 02/11] coresight: tmc-etr: Add barrier after updating AUX ring buffer
Date: Mon, 12 Jul 2021 11:40:12 +0100	[thread overview]
Message-ID: <f9734d69-b1f5-825c-f152-705626f4b256@arm.com> (raw)
In-Reply-To: <20210711104105.505728-3-leo.yan@linaro.org>

On 11/07/2021 11:40, Leo Yan wrote:
> Since a memory barrier is required between AUX trace data store and
> aux_head store, and the AUX trace data is filled with memcpy(), it's
> sufficient to use smp_wmb() so can ensure the trace data is visible
> prior to updating aux_head.
> 
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
>   drivers/hwtracing/coresight/coresight-tmc-etr.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index acdb59e0e661..713205db15a1 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -1563,6 +1563,14 @@ tmc_update_etr_buffer(struct coresight_device *csdev,
>   	 */
>   	if (etr_perf->snapshot)
>   		handle->head += size;
> +
> +	/*
> +	 * It requires the ordering between the AUX trace data and aux_head
> +	 * store, below smp_wmb() ensures the AUX trace data is visible prior
> +	 * to updating aux_head.
> +	 */

Please could we reword this a bit, something like :

	/*
	 * Ensure that the AUX trace data is visible before the aux_head
	 * is updated via perf_aux_output_end(), as expected by the
	 * perf ring buffer.
	 */
> +	smp_wmb();
> +

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>