coresight: etm-pmu: Ensure the AUX handle is valid

The ETM perf infrastructure closes out a handle during event_stop
or on an error in starting the event. In either case, it is possible
for a "sink" to update/close the handle, under certain circumstances.
(e.g no space in ring buffer.). So, ensure that we handle this
gracefully in the PMU driver by verifying the handle is still valid.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Leo Yan <leo.yan@linaro.org>
Reviewed-by: NAnshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20210914102641.1852544-4-suzuki.poulose@arm.comSigned-off-by: NMathieu Poirier <mathieu.poirier@linaro.org>

drivers/hwtracing/coresight/coresight-etm-perf.c

@@ -485,8 +485,15 @@ static void etm_event_start(struct perf_event *event, int flags)
 fail_disable_path:
 	coresight_disable_path(path);
 fail_end_stop:
-	perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
-	perf_aux_output_end(handle, 0);
+	/*
+	 * Check if the handle is still associated with the event,
+	 * to handle cases where if the sink failed to start the
+	 * trace and TRUNCATED the handle already.
+	 */
+	if (READ_ONCE(handle->event)) {
+		perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
+		perf_aux_output_end(handle, 0);
+	}
 fail:
 	event->hw.state = PERF_HES_STOPPED;
 	return;
@@ -567,7 +574,21 @@ static void etm_event_stop(struct perf_event *event, int mode)
 
 		size = sink_ops(sink)->update_buffer(sink, handle,
 					      event_data->snk_config);
-		perf_aux_output_end(handle, size);
+		/*
+		 * Make sure the handle is still valid as the
+		 * sink could have closed it from an IRQ.
+		 * The sink driver must handle the race with
+		 * update_buffer() and IRQ. Thus either we
+		 * should get a valid handle and valid size
+		 * (which may be 0).
+		 *
+		 * But we should never get a non-zero size with
+		 * an invalid handle.
+		 */
+		if (READ_ONCE(handle->event))
+			perf_aux_output_end(handle, size);
+		else
+			WARN_ON(size);
 	}
 
 	/* Disabling the path make its elements available to other sessions */