linux-kernel - [PATCH v5 1/9] perf/ring_buffer: Add comment for barriers on AUX ring buffer

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Message-Id: <20210809111407.596077-2-leo.yan@linaro.org>
Date:   Mon,  9 Aug 2021 19:13:59 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Michael Petlan <mpetlan@...hat.com>,
        "Frank Ch. Eigler" <fche@...hat.com>,
        Song Liu <songliubraving@...com>, x86@...nel.org,
        Daniel Díaz <daniel.diaz@...aro.org>,
        Andrii Nakryiko <andriin@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org
Cc:     Leo Yan <leo.yan@...aro.org>
Subject: [PATCH v5 1/9] perf/ring_buffer: Add comment for barriers on AUX ring buffer

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@...aro.org>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 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