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

Alexander Shishkin
2016-03-04T13:43:35Z

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@linux.intel.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.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