Implement the content/renderer and content/browser part of the Device Motion API.

content/common/one_writer_seqlock.h

Index: content/common/one_writer_seqlock.h
diff --git a/content/common/one_writer_seqlock.h b/content/common/one_writer_seqlock.h
new file mode 100644
index 0000000000000000000000000000000000000000..6774df6c0156a467041dd8c29445bcc85777cea5
--- /dev/null
+++ b/content/common/one_writer_seqlock.h
@@ -0,0 +1,46 @@
+// 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_ONE_WRITER_SEQLOCK_H_
+#define CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_
+
+#include "base/atomicops.h"
+#include "base/threading/platform_thread.h"
+#include "content/common/content_export.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 type of lock is used in two implementations (gamepad and
+// device motion, in particular see e.g. shared_memory_seqlock_buffer.h).
+// It may make sense to generalize this lock to multiple writers.
+//
+// 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 CONTENT_EXPORT OneWriterSeqLock {

[darin (slow to review), 2013/06/24 23:40:34]

nit: Since you didn't freshly write these files, consider using a lower
--similiarity option with "git cl upload".  That way history will be preserved.

[timvolodine, 2013/06/25 03:36:07]

Done.

+ public:
+  OneWriterSeqLock();
+  base::subtle::Atomic32 ReadBegin();
+  bool ReadRetry(base::subtle::Atomic32 version);
+  void WriteBegin();
+  void WriteEnd();
+
+ private:
+  base::subtle::Atomic32 sequence_;
+  DISALLOW_COPY_AND_ASSIGN(OneWriterSeqLock);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_