Revert 146792 - Lock-free GamepadSeqLock (try 2)

content/common/gamepad_seqlock.h

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// 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/basictypes.h"
+#include "base/threading/platform_thread.h"
 #include "content/common/content_export.h"
 
 namespace content {
 
-namespace internal {
-
-class CONTENT_EXPORT GamepadSeqLockBase {
- protected:
-  static const size_t kEntries = 2;
-  typedef base::subtle::Atomic32 Atomic32;
-  struct BaseEntry {
-    Atomic32 sequence_;
-    // Both writer and readers work with the array concurrently,
-    // so it must be accessed with atomic operations.
-    Atomic32 data_[1];
-  };
-
-  GamepadSeqLockBase(BaseEntry* entries, size_t size);
-  void ReadTo(Atomic32* obj);
-  void Write(const Atomic32* obj);
+// 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 CONTENT_EXPORT GamepadSeqLock {
+ public:
+  GamepadSeqLock();
+  base::subtle::Atomic32 ReadBegin();
+  bool ReadRetry(base::subtle::Atomic32 version);
+  void WriteBegin();
+  void WriteEnd();
 
  private:
-  const size_t size_;
-  Atomic32 current_;
-  BaseEntry* entries_[kEntries];
-};
-
-}  // namespace internal
-
-// This SeqLock handles only *one* writer and multiple readers. It may be
-// suitable for high read frequency scenarios and is especially useful in shared
-// memory environment. See for the basic idea:
-//   http://en.wikipedia.org/wiki/Seqlock
-// However, this implementation is an improved lock-free variant.
-// The SeqLock can hold only POD fully-embed data (no pointers
-// to satellite data), copies are done with memcpy.
-template<typename T>
-class GamepadSeqLock : public internal::GamepadSeqLockBase {
- public:
-  GamepadSeqLock()
-      : internal::GamepadSeqLockBase(entries_, kSize) {
-  }
-
-  // May be called concurrently with ReadTo and Write.
-  // - If ReadTo occurs in the middle of Write (on another thread),
-  //   ReadTo gets the old data (not the new data being written
-  //   by the other thread).
-  // - ReadTo never spins unless the writer thread is calling Write
-  //   multiple times before ReadTo completes.
-  void ReadTo(T* obj) {
-    // Breaks strict-aliasing rules.
-    Base::ReadTo(reinterpret_cast<Atomic32*>(obj));
-  }
-
-  // Must be called by a single thread at a time.  May be called concurrently
-  // with ReadTo.
-  void Write(const T& obj) {
-    // Breaks strict-aliasing rules.
-    Base::Write(reinterpret_cast<const Atomic32*>(&obj));
-  }
-
-private:
-  typedef internal::GamepadSeqLockBase Base;
-  COMPILE_ASSERT(sizeof(T) % sizeof(Atomic32) == 0, bad_object_size);
-  static size_t const kSize = sizeof(T) / sizeof(Atomic32);
-  struct Entry : Base::BaseEntry {
-    Atomic32 data_[kSize];
-  };
-  Entry entries_[kEntries];
+  base::subtle::Atomic32 sequence_;
   DISALLOW_COPY_AND_ASSIGN(GamepadSeqLock);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_GAMEPAD_SEQLOCK_H_