| // 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 |
| |
| #if defined(OS_WIN) |
| #include <windows.h> |
| #endif |
| |
| namespace base { |
| |
| 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. The wait functions are |
| // susceptible to spurious wakeups. (See usage note 1 for more details.) |
| void Wait(); |
| void TimedWait(const TimeDelta& max_time); |
| |
| // Broadcast() revives all waiting threads. (See usage note 2 for more |
| // details.) |
| void Broadcast(); |
| // Signal() revives one waiting thread. |
| void Signal(); |
| |
| private: |
| |
| #if defined(OS_WIN) |
| CONDITION_VARIABLE cv_; |
| SRWLOCK* const srwlock_; |
| #elif defined(OS_POSIX) |
| pthread_cond_t condition_; |
| pthread_mutex_t* user_mutex_; |
| #endif |
| |
| #if DCHECK_IS_ON() && (defined(OS_WIN) || defined(OS_POSIX)) |
| base::Lock* const user_lock_; // Needed to adjust shadow lock state on wait. |
| #endif |
| |
| DISALLOW_COPY_AND_ASSIGN(ConditionVariable); |
| }; |
| |
| } // namespace base |
| |
| #endif // BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ |