// Copyright 2006-2008 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// ConditionVariable wraps pthreads condition variable synchronization or, on
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// Windows, simulates it. This functionality is very helpful for having
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// several threads wait for an event, as is common with a thread pool managed
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// by a master. The meaning of such an event in the (worker) thread pool
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// scenario is that additional tasks are now available for processing. It is
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// used in Chrome in the DNS prefetching system to notify worker threads that
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// a queue now has items (tasks) which need to be tended to. A related use
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// would have a pool manager waiting on a ConditionVariable, waiting for a
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// thread in the pool to announce (signal) that there is now more room in a
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// (bounded size) communications queue for the manager to deposit tasks, or,
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// as a second example, that the queue of tasks is completely empty and all
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// workers are waiting.
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//
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// USAGE NOTE 1: spurious signal events are possible with this and
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// most implementations of condition variables. As a result, be
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// *sure* to retest your condition before proceeding. The following
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// is a good example of doing this correctly:
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//
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// while (!work_to_be_done()) Wait(...);
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//
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// In contrast do NOT do the following:
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//
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// if (!work_to_be_done()) Wait(...); // Don't do this.
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//
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// Especially avoid the above if you are relying on some other thread only
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// issuing a signal up *if* there is work-to-do. There can/will
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// be spurious signals. Recheck state on waiting thread before
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// assuming the signal was intentional. Caveat caller ;-).
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//
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// USAGE NOTE 2: Broadcast() frees up all waiting threads at once,
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// which leads to contention for the locks they all held when they
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// called Wait(). This results in POOR performance. A much better
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// approach to getting a lot of threads out of Wait() is to have each
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// thread (upon exiting Wait()) call Signal() to free up another
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// Wait'ing thread. Look at condition_variable_unittest.cc for
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// both examples.
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//
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// Broadcast() can be used nicely during teardown, as it gets the job
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// done, and leaves no sleeping threads... and performance is less
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// critical at that point.
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//
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// The semantics of Broadcast() are carefully crafted so that *all*
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// threads that were waiting when the request was made will indeed
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// get signaled. Some implementations mess up, and don't signal them
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// all, while others allow the wait to be effectively turned off (for
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// a while while waiting threads come around). This implementation
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// appears correct, as it will not "lose" any signals, and will guarantee
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// that all threads get signaled by Broadcast().
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//
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// This implementation offers support for "performance" in its selection of
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// which thread to revive. Performance, in direct contrast with "fairness,"
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// assures that the thread that most recently began to Wait() is selected by
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// Signal to revive. Fairness would (if publicly supported) assure that the
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// thread that has Wait()ed the longest is selected. The default policy
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// may improve performance, as the selected thread may have a greater chance of
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// having some of its stack data in various CPU caches.
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//
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// For a discussion of the many very subtle implementation details, see the FAQ
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// at the end of condition_variable_win.cc.
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#ifndef MINI_CHROMIUM_BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_
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#define MINI_CHROMIUM_BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_
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#include "build/build_config.h"
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#include <pthread.h>
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#include "base/macros.h"
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#include "base/synchronization/lock.h"
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namespace base {
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class ConditionVarImpl;
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class ConditionVariable {
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public:
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// Construct a cv for use with ONLY one user lock.
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explicit ConditionVariable(Lock* user_lock);
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~ConditionVariable();
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// Wait() releases the caller's critical section atomically as it starts to
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// sleep, and the reacquires it when it is signaled.
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void Wait();
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// Broadcast() revives all waiting threads.
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void Broadcast();
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// Signal() revives one waiting thread.
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void Signal();
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private:
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pthread_cond_t condition_;
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pthread_mutex_t* user_mutex_;
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#ifndef NDEBUG
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base::Lock* user_lock_; // Needed to adjust shadow lock state on wait.
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#endif
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DISALLOW_COPY_AND_ASSIGN(ConditionVariable);
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};
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} // namespace base
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#endif // MINI_CHROMIUM_BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_
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