Split default and user-supplied event tracers.

src/utils/SkEventTracer.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
+#include "SkAtomics.h"
 #include "SkEventTracer.h"
-#include "SkOnce.h"
+#include "SkLazyPtr.h"
 
-class SkDefaultEventTracer: public SkEventTracer {
+class SkDefaultEventTracer : public SkEventTracer {
     virtual SkEventTracer::Handle
         addTraceEvent(char phase,
                       const uint8_t* categoryEnabledFlag,
                       const char* name,
                       uint64_t id,
                       int numArgs,
                       const char** argNames,
                       const uint8_t* argTypes,
                       const uint64_t* argValues,
                       uint8_t flags) override { return 0; }
 
     virtual void
         updateTraceEventDuration(const uint8_t* categoryEnabledFlag,
                                  const char* name,
                                  SkEventTracer::Handle handle) override {};
 
     const uint8_t* getCategoryGroupEnabled(const char* name) override {
         static uint8_t no = 0;
         return &no;
     };
     virtual const char* getCategoryGroupName(
       const uint8_t* categoryEnabledFlag) override {
         static const char* dummy = "dummy";
         return dummy;
     };
 };
 
-SkEventTracer* SkEventTracer::gInstance;
+// We prefer gUserTracer if it's been set, otherwise we fall back on gDefaultTracer.
+static SkEventTracer* gUserTracer = nullptr;
+SK_DECLARE_STATIC_LAZY_PTR(SkDefaultEventTracer, gDefaultTracer);
 
-static void cleanup_tracer() {
-    // calling SetInstance will delete the existing instance.
-    SkEventTracer::SetInstance(NULL);
+// We can use relaxed memory order for gUserTracer loads and stores.
+// It's not guarding anything but itself.
+
+void SkEventTracer::SetInstance(SkEventTracer* tracer) {
+    SkASSERT(nullptr == sk_atomic_load(&gUserTracer, sk_memory_order_relaxed));
+    sk_atomic_store(&gUserTracer, tracer, sk_memory_order_relaxed);
+    // An atomic load during process shutdown is probably overkill, but safe overkill.
+    atexit([](){ SkDELETE(sk_atomic_load(&gUserTracer, sk_memory_order_relaxed)); });
 }
 
-static void intialize_default_tracer(SkEventTracer* current_instance) {
-    if (NULL == current_instance) {
-        SkEventTracer::SetInstance(SkNEW(SkDefaultEventTracer));
+SkEventTracer* SkEventTracer::GetInstance() {
+    if (SkEventTracer* tracer = sk_atomic_load(&gUserTracer, sk_memory_order_relaxed)) {

[Alexander Potapenko, 2015/04/21 15:55:38]

Since you're dereferincing this pointer, you need an acquire-load/release-store
pair from the C++ memory model POV.

+        return tracer;
     }
-    atexit(cleanup_tracer);
+    return gDefaultTracer.get();
 }
-
-
-SK_DECLARE_STATIC_ONCE(once);
-SkEventTracer* SkEventTracer::GetInstance() {
-    SkOnce(&once, intialize_default_tracer, SkEventTracer::gInstance);
-    SkASSERT(SkEventTracer::gInstance);
-    return SkEventTracer::gInstance;
-}