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 // ConditionVariable wraps pthreads condition variable synchronization or, on 6 // Windows, simulates it. This functionality is very helpful for having 7 // several threads wait for an event, as is common with a thread pool managed 8 // by a master. The meaning of such an event in the (worker) thread pool 9 // scenario is that additional tasks are now available for processing. It is 10 // used in Chrome in the DNS prefetching system to notify worker threads that 11 // a queue now has items (tasks) which need to be tended to. A related use 12 // would have a pool manager waiting on a ConditionVariable, waiting for a 13 // thread in the pool to announce (signal) that there is now more room in a 14 // (bounded size) communications queue for the manager to deposit tasks, or, 15 // as a second example, that the queue of tasks is completely empty and all 16 // workers are waiting. 17 // 18 // USAGE NOTE 1: spurious signal events are possible with this and 19 // most implementations of condition variables. As a result, be 20 // *sure* to retest your condition before proceeding. The following 21 // is a good example of doing this correctly: 22 // 23 // while (!work_to_be_done()) Wait(...); 24 // 25 // In contrast do NOT do the following: 26 // 27 // if (!work_to_be_done()) Wait(...); // Don't do this. 28 // 29 // Especially avoid the above if you are relying on some other thread only 30 // issuing a signal up *if* there is work-to-do. There can/will 31 // be spurious signals. Recheck state on waiting thread before 32 // assuming the signal was intentional. Caveat caller ;-). 33 // 34 // USAGE NOTE 2: Broadcast() frees up all waiting threads at once, 35 // which leads to contention for the locks they all held when they 36 // called Wait(). This results in POOR performance. A much better 37 // approach to getting a lot of threads out of Wait() is to have each 38 // thread (upon exiting Wait()) call Signal() to free up another 39 // Wait'ing thread. Look at condition_variable_unittest.cc for 40 // both examples. 41 // 42 // Broadcast() can be used nicely during teardown, as it gets the job 43 // done, and leaves no sleeping threads... and performance is less 44 // critical at that point. 45 // 46 // The semantics of Broadcast() are carefully crafted so that *all* 47 // threads that were waiting when the request was made will indeed 48 // get signaled. Some implementations mess up, and don't signal them 49 // all, while others allow the wait to be effectively turned off (for 50 // a while while waiting threads come around). This implementation 51 // appears correct, as it will not "lose" any signals, and will guarantee 52 // that all threads get signaled by Broadcast(). 53 // 54 // This implementation offers support for "performance" in its selection of 55 // which thread to revive. Performance, in direct contrast with "fairness," 56 // assures that the thread that most recently began to Wait() is selected by 57 // Signal to revive. Fairness would (if publicly supported) assure that the 58 // thread that has Wait()ed the longest is selected. The default policy 59 // may improve performance, as the selected thread may have a greater chance of 60 // having some of its stack data in various CPU caches. 61 // 62 // For a discussion of the many very subtle implementation details, see the FAQ 63 // at the end of condition_variable_win.cc. 64 65 #ifndef BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ 66 #define BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ 67 68 #include "base/base_export.h" 69 #include "base/logging.h" 70 #include "base/macros.h" 71 #include "base/synchronization/lock.h" 72 #include "build/build_config.h" 73 74 #if defined(OS_POSIX) 75 #include <pthread.h> 76 #endif 77 78 #if defined(OS_WIN) 79 #include <windows.h> 80 #endif 81 82 namespace base { 83 84 class TimeDelta; 85 86 class BASE_EXPORT ConditionVariable { 87 public: 88 // Construct a cv for use with ONLY one user lock. 89 explicit ConditionVariable(Lock* user_lock); 90 91 ~ConditionVariable(); 92 93 // Wait() releases the caller's critical section atomically as it starts to 94 // sleep, and the reacquires it when it is signaled. The wait functions are 95 // susceptible to spurious wakeups. (See usage note 1 for more details.) 96 void Wait(); 97 void TimedWait(const TimeDelta& max_time); 98 99 // Broadcast() revives all waiting threads. (See usage note 2 for more 100 // details.) 101 void Broadcast(); 102 // Signal() revives one waiting thread. 103 void Signal(); 104 105 private: 106 107 #if defined(OS_WIN) 108 CONDITION_VARIABLE cv_; 109 SRWLOCK* const srwlock_; 110 #elif defined(OS_POSIX) 111 pthread_cond_t condition_; 112 pthread_mutex_t* user_mutex_; 113 #endif 114 115 #if DCHECK_IS_ON() && (defined(OS_WIN) || defined(OS_POSIX)) 116 base::Lock* const user_lock_; // Needed to adjust shadow lock state on wait. 117 #endif 118 119 DISALLOW_COPY_AND_ASSIGN(ConditionVariable); 120 }; 121 122 } // namespace base 123 124 #endif // BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ 125
