// Copyright (c) 2011 The Chromium Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. // ConditionVariable wraps pthreads condition variable synchronization or, on // Windows, simulates it. This functionality is very helpful for having // several threads wait for an event, as is common with a thread pool managed // by a master. The meaning of such an event in the (worker) thread pool // scenario is that additional tasks are now available for processing. It is // used in Chrome in the DNS prefetching system to notify worker threads that // a queue now has items (tasks) which need to be tended to. A related use // would have a pool manager waiting on a ConditionVariable, waiting for a // thread in the pool to announce (signal) that there is now more room in a // (bounded size) communications queue for the manager to deposit tasks, or, // as a second example, that the queue of tasks is completely empty and all // workers are waiting. // // USAGE NOTE 1: spurious signal events are possible with this and // most implementations of condition variables. As a result, be // *sure* to retest your condition before proceeding. The following // is a good example of doing this correctly: // // while (!work_to_be_done()) Wait(...); // // In contrast do NOT do the following: // // if (!work_to_be_done()) Wait(...); // Don't do this. // // Especially avoid the above if you are relying on some other thread only // issuing a signal up *if* there is work-to-do. There can/will // be spurious signals. Recheck state on waiting thread before // assuming the signal was intentional. Caveat caller ;-). // // USAGE NOTE 2: Broadcast() frees up all waiting threads at once, // which leads to contention for the locks they all held when they // called Wait(). This results in POOR performance. A much better // approach to getting a lot of threads out of Wait() is to have each // thread (upon exiting Wait()) call Signal() to free up another // Wait'ing thread. Look at condition_variable_unittest.cc for // both examples. // // Broadcast() can be used nicely during teardown, as it gets the job // done, and leaves no sleeping threads... and performance is less // critical at that point. // // The semantics of Broadcast() are carefully crafted so that *all* // threads that were waiting when the request was made will indeed // get signaled. Some implementations mess up, and don't signal them // all, while others allow the wait to be effectively turned off (for // a while while waiting threads come around). This implementation // appears correct, as it will not "lose" any signals, and will guarantee // that all threads get signaled by Broadcast(). // // This implementation offers support for "performance" in its selection of // which thread to revive. Performance, in direct contrast with "fairness," // assures that the thread that most recently began to Wait() is selected by // Signal to revive. Fairness would (if publicly supported) assure that the // thread that has Wait()ed the longest is selected. The default policy // may improve performance, as the selected thread may have a greater chance of // having some of its stack data in various CPU caches. // // For a discussion of the many very subtle implementation details, see the FAQ // at the end of condition_variable_win.cc. #ifndef BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ #define BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ #include "base/base_export.h" #include "base/logging.h" #include "base/macros.h" #include "base/synchronization/lock.h" #include "build/build_config.h" #if defined(OS_POSIX) #include <pthread.h> #endif namespace base { class ConditionVarImpl; class TimeDelta; class BASE_EXPORT ConditionVariable { public: // Construct a cv for use with ONLY one user lock. explicit ConditionVariable(Lock* user_lock); ~ConditionVariable(); // Wait() releases the caller's critical section atomically as it starts to // sleep, and the reacquires it when it is signaled. void Wait(); void TimedWait(const TimeDelta& max_time); // Broadcast() revives all waiting threads. void Broadcast(); // Signal() revives one waiting thread. void Signal(); private: #if defined(OS_WIN) ConditionVarImpl* impl_; #elif defined(OS_POSIX) pthread_cond_t condition_; pthread_mutex_t* user_mutex_; #if DCHECK_IS_ON() base::Lock* user_lock_; // Needed to adjust shadow lock state on wait. #endif #endif DISALLOW_COPY_AND_ASSIGN(ConditionVariable); }; } // namespace base #endif // BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_