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#include "content/common/one_writer_seqlock.h"
6
7namespace content {
8
9OneWriterSeqLock::OneWriterSeqLock()
10  : sequence_(0) {
11}
12
13base::subtle::Atomic32 OneWriterSeqLock::ReadBegin() {
14  base::subtle::Atomic32 version;
15  for (;;) {
16    version = base::subtle::NoBarrier_Load(&sequence_);
17
18    // If the counter is even, then the associated data might be in a
19    // consistent state, so we can try to read.
20    if ((version & 1) == 0)
21      break;
22
23    // Otherwise, the writer is in the middle of an update. Retry the read.
24    base::PlatformThread::YieldCurrentThread();
25  }
26  return version;
27}
28
29bool OneWriterSeqLock::ReadRetry(base::subtle::Atomic32 version) {
30  // If the sequence number was updated then a read should be re-attempted.
31  // -- Load fence, read membarrier
32  return base::subtle::Release_Load(&sequence_) != version;
33}
34
35void OneWriterSeqLock::WriteBegin() {
36  // Increment the sequence number to odd to indicate the beginning of a write
37  // update.
38  base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
39  // -- Store fence, write membarrier
40}
41
42void OneWriterSeqLock::WriteEnd() {
43  // Increment the sequence to an even number to indicate the completion of
44  // a write update.
45  // -- Store fence, write membarrier
46  base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
47}
48
49} // namespace content
50