content/common/one_writer_seqlock.h

   1 // Copyright 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_
   6 #define CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_
   7
   8 #include "base/atomicops.h"
   9 #include "base/threading/platform_thread.h"
  10 #include "content/common/content_export.h"
  11
  12 namespace content {
  13
  14 // This SeqLock handles only *one* writer and multiple readers. It may be
  15 // suitable for low-contention with relatively infrequent writes, and many
  16 // readers. See:
  17 //   http://en.wikipedia.org/wiki/Seqlock
  18 //   http://www.concurrencykit.org/doc/ck_sequence.html
  19 // This implementation is based on ck_sequence.h from http://concurrencykit.org.
  20 //
  21 // Currently this type of lock is used in two implementations (gamepad and
  22 // device motion, in particular see e.g. shared_memory_seqlock_buffer.h).
  23 // It may make sense to generalize this lock to multiple writers.
  24 //
  25 // You must be very careful not to operate on potentially inconsistent read
  26 // buffers. If the read must be retry'd, the data in the read buffer could
  27 // contain any random garbage. e.g., contained pointers might be
  28 // garbage, or indices could be out of range. Probably the only suitable thing
  29 // to do during the read loop is to make a copy of the data, and operate on it
  30 // only after the read was found to be consistent.
  31 class CONTENT_EXPORT OneWriterSeqLock {
  32  public:
  33   OneWriterSeqLock();
  34   base::subtle::Atomic32 ReadBegin();
  35   bool ReadRetry(base::subtle::Atomic32 version);
  36   void WriteBegin();
  37   void WriteEnd();
  38
  39  private:
  40   base::subtle::Atomic32 sequence_;
  41   DISALLOW_COPY_AND_ASSIGN(OneWriterSeqLock);
  42 };
  43
  44 }  // namespace content
  45
  46 #endif  // CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_