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// 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_
#pragma once
#include "build/build_config.h"
#if defined(OS_WIN)
#include <windows.h>
#elif defined(OS_POSIX)
#include <pthread.h>
#endif
#include "base/basictypes.h"
#include "base/synchronization/lock.h"
namespace base {
class TimeDelta;
class 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)
// Define Event class that is used to form circularly linked lists.
// The list container is an element with NULL as its handle_ value.
// The actual list elements have a non-zero handle_ value.
// All calls to methods MUST be done under protection of a lock so that links
// can be validated. Without the lock, some links might asynchronously
// change, and the assertions would fail (as would list change operations).
class Event {
public:
// Default constructor with no arguments creates a list container.
Event();
~Event();
// InitListElement transitions an instance from a container, to an element.
void InitListElement();
// Methods for use on lists.
bool IsEmpty() const;
void PushBack(Event* other);
Event* PopFront();
Event* PopBack();
// Methods for use on list elements.
// Accessor method.
HANDLE handle() const;
// Pull an element from a list (if it's in one).
Event* Extract();
// Method for use on a list element or on a list.
bool IsSingleton() const;
private:
// Provide pre/post conditions to validate correct manipulations.
bool ValidateAsDistinct(Event* other) const;
bool ValidateAsItem() const;
bool ValidateAsList() const;
bool ValidateLinks() const;
HANDLE handle_;
Event* next_;
Event* prev_;
DISALLOW_COPY_AND_ASSIGN(Event);
};
// Note that RUNNING is an unlikely number to have in RAM by accident.
// This helps with defensive destructor coding in the face of user error.
enum RunState { SHUTDOWN = 0, RUNNING = 64213 };
// Internal implementation methods supporting Wait().
Event* GetEventForWaiting();
void RecycleEvent(Event* used_event);
RunState run_state_;
// Private critical section for access to member data.
base::Lock internal_lock_;
// Lock that is acquired before calling Wait().
base::Lock& user_lock_;
// Events that threads are blocked on.
Event waiting_list_;
// Free list for old events.
Event recycling_list_;
int recycling_list_size_;
// The number of allocated, but not yet deleted events.
int allocation_counter_;
#elif defined(OS_POSIX)
pthread_cond_t condition_;
pthread_mutex_t* user_mutex_;
#if !defined(NDEBUG)
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_
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