use acquire/release in SkEventTracer.cpp

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 "SkLazyPtr.h"
@@ skipping 23 matching lines @@
       const uint8_t* categoryEnabledFlag) override {
         static const char* dummy = "dummy";
         return dummy;
     };
 };
 
 // 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);
 
-// 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);
+    SkASSERT(nullptr == sk_atomic_load(&gUserTracer, sk_memory_order_acquire));

[Alexander Potapenko, 2015/04/22 17:47:50]

I may be mistaking, but this one can be demoted to a nobarrier read, since
you're not using the value.

[mtklein, 2015/04/22 18:56:24]

Yeah, seemed simpler to keep it all as _acquire so I don't get confused and try
to make things all _relaxed again, especially because it's just a debug-only
assert.

+    sk_atomic_store(&gUserTracer, tracer, sk_memory_order_release);
     // An atomic load during process shutdown is probably overkill, but safe overkill.
-    atexit([](){ SkDELETE(sk_atomic_load(&gUserTracer, sk_memory_order_relaxed)); });
+    atexit([](){ SkDELETE(sk_atomic_load(&gUserTracer, sk_memory_order_acquire)); });
 }
 
 SkEventTracer* SkEventTracer::GetInstance() {
-    if (SkEventTracer* tracer = sk_atomic_load(&gUserTracer, sk_memory_order_relaxed)) {
+    if (SkEventTracer* tracer = sk_atomic_load(&gUserTracer, sk_memory_order_acquire)) {
         return tracer;
     }
     return gDefaultTracer.get();
 }