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Date:   Sun, 29 Aug 2021 18:51:30 +0800
From:   Leo Yan <leo.yan@linaro.org>
To:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        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>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Mike Leach <mike.leach@linaro.org>,
        Michael Petlan <mpetlan@redhat.com>,
        "Frank Ch. Eigler" <fche@redhat.com>,
        Song Liu <songliubraving@fb.com>, x86@kernel.org,
        Daniel =?iso-8859-1?Q?D=EDaz?= <daniel.diaz@linaro.org>,
        Andrii Nakryiko <andriin@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        coresight@lists.linaro.org
Subject: Re: [PATCH v5 1/9] perf/ring_buffer: Add comment for barriers on AUX
 ring buffer
Message-ID: <20210829105130.GA14461@leoy-ThinkPad-X240s>
References: <20210809111407.596077-1-leo.yan@linaro.org>
 <20210809111407.596077-2-leo.yan@linaro.org>
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In-Reply-To: <20210809111407.596077-2-leo.yan@linaro.org>
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Hi Peter,

On Mon, Aug 09, 2021 at 07:13:59PM +0800, Leo Yan wrote:
> AUX ring buffer applies almost the same barriers as perf ring buffer,
> but there has an exception for ordering between writing the AUX trace
> data and updating user_page::aux_head.
> 
> This patch adds comment for how to use the barriers on AUX ring buffer,
> and gives comment to ask the drivers to flush the trace data into AUX
> ring buffer prior to updating user_page::aux_head.
> 
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

You have given the ACK tag before, could you pick up this patch?

Thanks,
Leo

> ---
>  kernel/events/ring_buffer.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 52868716ec35..5cf6579be05e 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -509,6 +509,15 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
>  		perf_event_aux_event(handle->event, aux_head, size,
>  				     handle->aux_flags);
>  
> +	/*
> +	 * See perf_output_put_handle(), AUX ring buffer applies the same
> +	 * barrier pairing as the perf ring buffer; except for B, since
> +	 * AUX ring buffer is written by hardware trace, we cannot simply
> +	 * use the generic memory barrier (like smp_wmb()) prior to update
> +	 * user_page::aux_head, the hardware trace driver takes the
> +	 * responsibility to ensure the trace data has been flushed into
> +	 * the AUX buffer before calling perf_aux_output_end().
> +	 */
>  	WRITE_ONCE(rb->user_page->aux_head, rb->aux_head);
>  	if (rb_need_aux_wakeup(rb))
>  		wakeup = true;
> -- 
> 2.25.1
>