Renderer reading side of gamepad

content/common/gamepad_seqlock.h

Index: content/common/gamepad_seqlock.h
diff --git a/content/common/gamepad_seqlock.h b/content/common/gamepad_seqlock.h
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+// 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.
+
+#ifndef CONTENT_COMMON_GAMEPAD_SEQLOCK_H_
+#define CONTENT_COMMON_GAMEPAD_SEQLOCK_H_
+
+#include "base/atomicops.h"
+#include "base/threading/platform_thread.h"
+
+namespace content {
+
+// This SeqLock handles only *one* writer and multiple readers. It may be
+// suitable for low-contention with relatively infrequent writes, and many
+// readers. See:
+//   http://en.wikipedia.org/wiki/Seqlock
+//   http://www.concurrencykit.org/doc/ck_sequence.html
+// This implementation is based on ck_sequence.h from http://concurrencykit.org.
+//
+// Currently, this is used in only one location. It may make sense to
+// generalize with a higher-level construct that owns both the lock and the
+// data buffer, if it is to be used more widely.
+//
+// You must be very careful not to operate on potentially inconsistent read
+// buffers. If the read must be retry'd, the data in the read buffer could
+// contain any random garbage. e.g., contained pointers might be
+// garbage, or indices could be out of range. Probably the only suitable thing
+// to do during the read loop is to make a copy of the data, and operate on it
+// only after the read was found to be consistent.
+class GamepadSeqLock {
+ public:
+  GamepadSeqLock();
+  base::subtle::Atomic32 ReadBegin();
+  bool ReadRetry(base::subtle::Atomic32 version);
+  void WriteBegin();
+  void WriteEnd();
+
+ private:
+  base::subtle::Atomic32 sequence_;
+  DISALLOW_COPY_AND_ASSIGN(GamepadSeqLock);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_GAMEPAD_SEQLOCK_H_