content/common/one_writer_seqlock.cc

   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
   7 namespace content {
   8
   9 OneWriterSeqLock::OneWriterSeqLock()
  10   : sequence_(0) {
  11 }
  12
  13 base::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
  29 bool 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
  35 void 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
  42 void 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