Fix base::internal::Singleton to use acquire loads instead of nobarrier ones.

base/memory/singleton.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // PLEASE READ: Do you really need a singleton?
 //
 // Singletons make it hard to determine the lifetime of an object, which can
 // lead to buggy code and spurious crashes.
 //
 // Instead of adding another singleton into the mix, try to identify either:
@@ skipping 215 matching lines @@
   // member.
 
   // Return a pointer to the one true instance of the class.
   static Type* get() {
 #ifndef NDEBUG
     // Avoid making TLS lookup on release builds.
     if (!Traits::kAllowedToAccessOnNonjoinableThread)
       base::ThreadRestrictions::AssertSingletonAllowed();
 #endif
 
-    base::subtle::AtomicWord value = base::subtle::NoBarrier_Load(&instance_);
+    // The load has acquire memory ordering as the thread which reads the
+    // instance_ pointer must acquire visibility over the singleton data.
+    base::subtle::AtomicWord value = base::subtle::Acquire_Load(&instance_);
     if (value != 0 && value != base::internal::kBeingCreatedMarker) {
       // See the corresponding HAPPENS_BEFORE below.
       ANNOTATE_HAPPENS_AFTER(&instance_);
       return reinterpret_cast<Type*>(value);
     }
 
     // Object isn't created yet, maybe we will get to create it, let's try...
     if (base::subtle::Acquire_CompareAndSwap(
           &instance_, 0, base::internal::kBeingCreatedMarker) == 0) {
       // instance_ was NULL and is now kBeingCreatedMarker.  Only one thread
       // will ever get here.  Threads might be spinning on us, and they will
       // stop right after we do this store.
       Type* newval = Traits::New();
 
       // This annotation helps race detectors recognize correct lock-less
       // synchronization between different threads calling get().
       // See the corresponding HAPPENS_AFTER below and above.
       ANNOTATE_HAPPENS_BEFORE(&instance_);
+      // Releases the visibility over instance_ to the readers.
       base::subtle::Release_Store(
           &instance_, reinterpret_cast<base::subtle::AtomicWord>(newval));
 
       if (newval != NULL && Traits::kRegisterAtExit)
         base::AtExitManager::RegisterCallback(OnExit, NULL);
 
       return newval;
     }
 
     // We hit a race. Wait for the other thread to complete it.
@@ skipping 15 matching lines @@
     instance_ = 0;
   }
   static base::subtle::AtomicWord instance_;
 };
 
 template <typename Type, typename Traits, typename DifferentiatingType>
 base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
     instance_ = 0;
 
 #endif  // BASE_MEMORY_SINGLETON_H_