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| 1 // Copyright 2016 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 BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_ |
| 6 #define BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_ |
| 7 |
| 8 #include "base/base_export.h" |
| 9 #include "base/macros.h" |
| 10 #include "build/build_config.h" |
| 11 |
| 12 #if defined(OS_NACL) |
| 13 #include "base/synchronization/lock.h" |
| 14 #endif |
| 15 |
| 16 #if defined(OS_WIN) |
| 17 #include <windows.h> |
| 18 #elif defined(OS_POSIX) |
| 19 #include <pthread.h> |
| 20 #else |
| 21 # error No reader-writer lock defined for this platform. |
| 22 #endif |
| 23 |
| 24 namespace base { |
| 25 namespace subtle { |
| 26 |
| 27 // An OS-independent wrapper around reader-writer locks. There's no magic here. |
| 28 // |
| 29 // You are strongly encouraged to use base::Lock instead of this, unless you |
| 30 // can demonstrate contention and show that this would lead to an improvement. |
| 31 // This lock does not make any guarantees of fairness, which can lead to writer |
| 32 // starvation under certain access patterns. You should carefully consider your |
| 33 // writer access patterns before using this lock. |
| 34 class BASE_EXPORT ReadWriteLock { |
| 35 public: |
| 36 ReadWriteLock(); |
| 37 ~ReadWriteLock(); |
| 38 |
| 39 // Reader lock functions. |
| 40 void ReadAcquire(); |
| 41 void ReadRelease(); |
| 42 |
| 43 // Writer lock functions. |
| 44 void WriteAcquire(); |
| 45 void WriteRelease(); |
| 46 |
| 47 private: |
| 48 #if defined(OS_WIN) |
| 49 using NativeHandle = SRWLOCK; |
| 50 #elif defined(OS_NACL) |
| 51 using NativeHandle = Lock; |
| 52 #elif defined(OS_POSIX) |
| 53 using NativeHandle = pthread_rwlock_t; |
| 54 #endif |
| 55 |
| 56 NativeHandle native_handle_; |
| 57 |
| 58 #if defined(OS_NACL) |
| 59 // Even though NaCl has a pthread_rwlock implementation, the build rules don't |
| 60 // make it universally available. So instead, implement a slower and trivial |
| 61 // reader-writer lock using a regular mutex. |
| 62 // TODO(amistry): Remove this and use the posix implementation when it's |
| 63 // available in all build configurations. |
| 64 uint32_t readers_ = 0; |
| 65 // base::Lock does checking to ensure the lock is acquired and released on the |
| 66 // same thread. This is not the case for this lock, so use pthread mutexes |
| 67 // directly here. |
| 68 pthread_mutex_t writer_lock_ = PTHREAD_MUTEX_INITIALIZER; |
| 69 #endif |
| 70 |
| 71 DISALLOW_COPY_AND_ASSIGN(ReadWriteLock); |
| 72 }; |
| 73 |
| 74 class AutoReadLock { |
| 75 public: |
| 76 explicit AutoReadLock(ReadWriteLock& lock) : lock_(lock) { |
| 77 lock_.ReadAcquire(); |
| 78 } |
| 79 ~AutoReadLock() { |
| 80 lock_.ReadRelease(); |
| 81 } |
| 82 |
| 83 private: |
| 84 ReadWriteLock& lock_; |
| 85 DISALLOW_COPY_AND_ASSIGN(AutoReadLock); |
| 86 }; |
| 87 |
| 88 class AutoWriteLock { |
| 89 public: |
| 90 explicit AutoWriteLock(ReadWriteLock& lock) : lock_(lock) { |
| 91 lock_.WriteAcquire(); |
| 92 } |
| 93 ~AutoWriteLock() { |
| 94 lock_.WriteRelease(); |
| 95 } |
| 96 |
| 97 private: |
| 98 ReadWriteLock& lock_; |
| 99 DISALLOW_COPY_AND_ASSIGN(AutoWriteLock); |
| 100 }; |
| 101 |
| 102 } // namespace subtle |
| 103 } // namespace base |
| 104 |
| 105 #endif // BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_ |
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