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

The NoBarrier_Load operations don't actually guarantee that the singleton data
associated with the pointer is visible to the thread calling get() even if the
pointer is initialized.

R=mark@chromium.org, dvyukov@chromium.org, epenner@chromium.org

Review URL: https://codereview.chromium.org/188603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255552 0039d316-1c4b-4281-b951-d872f2087c98

base/memory/singleton.cc

@@ -18,7 +18,10 @@ subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance) {
   // the object has been created.
   subtle::AtomicWord value;
   while (true) {
-    value = subtle::NoBarrier_Load(instance);
+    // The load has acquire memory ordering as the thread which reads the
+    // instance pointer must acquire visibility over the associated data.
+    // The pairing Release_Store operation is in Singleton::get().
+    value = subtle::Acquire_Load(instance);
     if (value != kBeingCreatedMarker)
       break;
     PlatformThread::YieldCurrentThread();

base/memory/singleton.h

@@ -233,7 +233,9 @@ class Singleton {
       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_);
@@ -252,6 +254,7 @@ class Singleton {
       // 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));