linux-kernel - [PATCH v2 3/5] perf: Document aux api usage

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Date:	Fri,  4 Mar 2016 15:42:47 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: [PATCH v2 3/5] perf: Document aux api usage

In order to ensure safe aux buffer management, we rely on the assumption
that pmu::stop() stops its ongoing aux transaction and not just the hw.

This patch documents this requirement for perf_aux_output_{begin,end}()
apis.

Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
---
 kernel/events/ring_buffer.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index fb7f5bce27..b5feb7ebb9 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -252,6 +252,10 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
  * The ordering is similar to that of perf_output_{begin,end}, with
  * the exception of (B), which should be taken care of by the pmu
  * driver, since ordering rules will differ depending on hardware.
+ *
+ * Call this from pmu::start(); see the comment in perf_aux_output_end()
+ * about its use in pmu callbacks. Both can also be called from the PMI
+ * handler if needed.
  */
 void *perf_aux_output_begin(struct perf_output_handle *handle,
 			    struct perf_event *event)
@@ -323,6 +327,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
 	return handle->rb->aux_priv;
 
 err_put:
+	/* can't be last */
 	rb_free_aux(rb);
 
 err:
@@ -337,6 +342,10 @@ err:
  * aux_head and posting a PERF_RECORD_AUX into the perf buffer. It is the
  * pmu driver's responsibility to observe ordering rules of the hardware,
  * so that all the data is externally visible before this is called.
+ *
+ * Note: this has to be called from pmu::stop() callback, as the assumption
+ * of the aux buffer management code is that after pmu::stop(), the aux
+ * transaction must be stopped and therefore drop the aux reference count.
  */
 void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size,
 			 bool truncated)
@@ -376,6 +385,7 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size,
 	handle->event = NULL;
 
 	local_set(&rb->aux_nest, 0);
+	/* can't be last */
 	rb_free_aux(rb);
 	ring_buffer_put(rb);
 }
-- 
2.7.0