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| 1 // Copyright (c) 2011 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 |
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