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 50
