TSAN: use somewhat pithier SK_ANNOTATE_UNPROTECTED_READ.

include/core/SkDynamicAnnotations.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkDynamicAnnotations_DEFINED
+#define SkDynamicAnnotations_DEFINED
+
+// This file contains macros used to send out-of-band signals to dynamic instrumentation systems,
+// namely thread sanitizer.  This is a cut-down version of the full dynamic_annotations library with
+// only the features used by Skia.
+
+// We check the same define to know to enable the annotations, but prefix all our macros with SK_.
+#if DYNAMIC_ANNOTATIONS_ENABLED
+
+extern "C" {
+// TSAN provides these hooks.
+void AnnotateIgnoreReadsBegin(const char* file, int line);
+void AnnotateIgnoreReadsEnd(const char* file, int line);
+}  // extern "C"
+
+// SK_ANNOTATE_UNPROTECTED_READ can wrap any variable read to tell TSAN to ignore that it appears to
+// be a racy read.  This should be used only when we can make an external guarantee that though this
+// particular read is racy, it is being used as part of a mechanism which is thread safe.  Examples:
+//   - the first check in double-checked locking;
+//   - checking if a ref count is equal to 1.
+// Note that in both these cases, we must still add terrifyingly subtle memory barriers to provide
+// that overall thread safety guarantee.  Using this macro to shut TSAN up without providing such an
+// external guarantee is pretty much never correct.
+template <typename T>
+inline T SK_ANNOTATE_UNPROTECTED_READ(const volatile T& x) {
+    AnnotateIgnoreReadsBegin(__FILE__, __LINE__);
+    T read = x;
+    AnnotateIgnoreReadsEnd(__FILE__, __LINE__);
+    return read;
+}
+
+#else  // !DYNAMIC_ANNOTATIONS_ENABLED
+
+#define SK_ANNOTATE_UNPROTECTED_READ(x) (x)
+
+#endif
+
+#endif//SkDynamicAnnotations_DEFINED