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_ 47
