drm/i915/perf: Mark up the racy use of perf->exclusive_stream

Inside the general i915_oa_init_reg_state() we avoid using the
perf->mutex. However, we rely on perf->exclusive_stream being valid to
access at that point, and for that we have to control the race with
disabling perf. This relies on the disabling being a heavy barrier that
inspects all active contexts, after marking the perf->exclusive_stream
as not available. This should ensure that there are no more concurrent
accesses to the perf->exclusive_stream as we destroy it.

Mark up the races around the perf->exclusive_stream so that they stand
out much more. (And hopefully we will be running kcsan to start
validating that the only races we have are carefully controlled.)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227085723.1961649-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_perf.c

@@ -1405,8 +1405,10 @@ static void i915_oa_stream_destroy(struct i915_perf_stream *stream)
 	/*
 	 * Unset exclusive_stream first, it will be checked while disabling
 	 * the metric set on gen8+.
+	 *
+	 * See i915_oa_init_reg_state() and lrc_configure_all_contexts()
 	 */
-	perf->exclusive_stream = NULL;
+	WRITE_ONCE(perf->exclusive_stream, NULL);
 	perf->ops.disable_metric_set(stream);
 
 	free_oa_buffer(stream);
@@ -2847,7 +2849,7 @@ static int i915_oa_stream_init(struct i915_perf_stream *stream,
 		goto err_oa_buf_alloc;
 
 	stream->ops = &i915_oa_stream_ops;
-	perf->exclusive_stream = stream;
+	WRITE_ONCE(perf->exclusive_stream, stream);
 
 	ret = perf->ops.enable_metric_set(stream);
 	if (ret) {
@@ -2867,7 +2869,7 @@ static int i915_oa_stream_init(struct i915_perf_stream *stream,
 	return 0;
 
 err_enable:
-	perf->exclusive_stream = NULL;
+	WRITE_ONCE(perf->exclusive_stream, NULL);
 	perf->ops.disable_metric_set(stream);
 
 	free_oa_buffer(stream);
@@ -2893,12 +2895,11 @@ void i915_oa_init_reg_state(const struct intel_context *ce,
 {
 	struct i915_perf_stream *stream;
 
-	/* perf.exclusive_stream serialised by lrc_configure_all_contexts() */
-
 	if (engine->class != RENDER_CLASS)
 		return;
 
-	stream = engine->i915->perf.exclusive_stream;
+	/* perf.exclusive_stream serialised by lrc_configure_all_contexts() */
+	stream = READ_ONCE(engine->i915->perf.exclusive_stream);
 	/*
 	 * For gen12, only CTX_R_PWR_CLK_STATE needs update, but the caller
 	 * is already doing that, so nothing to be done for gen12 here.