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 78namespace base { 79 80class ConditionVarImpl; 81class TimeDelta; 82 83class BASE_EXPORT ConditionVariable { 84 public: 85 // Construct a cv for use with ONLY one user lock. 86 explicit ConditionVariable(Lock* user_lock); 87 88 ~ConditionVariable(); 89 90 // Wait() releases the caller's critical section atomically as it starts to 91 // sleep, and the reacquires it when it is signaled. 92 void Wait(); 93 void TimedWait(const TimeDelta& max_time); 94 95 // Broadcast() revives all waiting threads. 96 void Broadcast(); 97 // Signal() revives one waiting thread. 98 void Signal(); 99 100 private: 101 102#if defined(OS_WIN) 103 ConditionVarImpl* impl_; 104#elif defined(OS_POSIX) 105 pthread_cond_t condition_; 106 pthread_mutex_t* user_mutex_; 107#if DCHECK_IS_ON() 108 base::Lock* user_lock_; // Needed to adjust shadow lock state on wait. 109#endif 110 111#endif 112 113 DISALLOW_COPY_AND_ASSIGN(ConditionVariable); 114}; 115 116} // namespace base 117 118#endif // BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ 119