Renderer reading side of gamepad

content/common/gamepad_seqlock.cc

Index: content/common/gamepad_seqlock.cc
diff --git a/content/common/gamepad_seqlock.cc b/content/common/gamepad_seqlock.cc
new file mode 100644
index 0000000000000000000000000000000000000000..7f7e14f47e48b120fc70fe02fc4fa73737511216
--- /dev/null
+++ b/content/common/gamepad_seqlock.cc
@@ -0,0 +1,49 @@
+// 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.
+
+#include "content/common/gamepad_seqlock.h"
+
+namespace content {
+
+GamepadSeqLock::GamepadSeqLock()
+  : sequence_(0) {
+}
+
+base::subtle::Atomic32 GamepadSeqLock::ReadBegin() {
+  base::subtle::Atomic32 version;
+  for (;;) {
+    version = base::subtle::NoBarrier_Load(&sequence_);
+
+    // If the counter is even, then the associated data might be in a
+    // consistent state, so we can try to read.
+    if ((version & 1) == 0)
+      break;
+
+    // Otherwise, the writer is in the middle of an update. Retry the read.
+    base::PlatformThread::YieldCurrentThread();
+  }
+  return version;
+}
+
+bool GamepadSeqLock::ReadRetry(base::subtle::Atomic32 version) {
+  // If the sequence number was updated then a read should be re-attempted.
+  // -- Load fence, read membarrier
+  return base::subtle::Release_Load(&sequence_) != version;
+}
+
+void GamepadSeqLock::WriteBegin() {
+  // Increment the sequence number to odd to indicate the beginning of a write
+  // update.
+  base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
+  // -- Store fence, write membarrier
+}
+
+void GamepadSeqLock::WriteEnd() {
+  // Increment the sequence to an even number to indicate the completion of
+  // a write update.
+  // -- Store fence, write membarrier
+  base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
+}
+
+} // namespace content