Source/WTF/wtf/Condition.h - WebKit - Git at Google

 /*
  * Copyright (C) 2015-2016 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

 #ifndef WTF_Condition_h
 #define WTF_Condition_h

 #include <wtf/Noncopyable.h>
 #include <wtf/ParkingLot.h>
 #include <wtf/TimeWithDynamicClockType.h>

 namespace WTF {

 // This is a condition variable that is suitable for use with any lock-like object, including
 // our own WTF::Lock. It features standard wait()/notifyOne()/notifyAll() methods in addition to
 // a variety of wait-with-timeout methods. This includes methods that use WTF's own notion of
 // time, like wall-clock time (i.e. WallTime) and monotonic time (i.e. MonotonicTime). This is
 // a very efficient condition variable. It only requires one byte of memory. notifyOne() and
 // notifyAll() require just a load and branch for the fast case where no thread is waiting.
 // This condition variable, when used with WTF::Lock, can outperform a system condition variable
 // and lock by up to 58x.
 class Condition {
     WTF_MAKE_NONCOPYABLE(Condition);
 public:
     // Condition will accept any kind of time and convert it internally, but this typedef tells
     // you what kind of time Condition would be able to use without conversions. However, if you
     // are unlikely to be affected by the cost of conversions, it is better to use MonotonicTime.
     using Time = ParkingLot::Time;

     constexpr Condition() = default;

     // Wait on a parking queue while releasing the given lock. It will unlock the lock just before
     // parking, and relock it upon wakeup. Returns true if we woke up due to some call to
     // notifyOne() or notifyAll(). Returns false if we woke up due to a timeout. Note that this form
     // of waitUntil() has some quirks:
     //
     // No spurious wake-up: in order for this to return before the timeout, some notifyOne() or
     // notifyAll() call must have happened. No scenario other than timeout or notify can lead to this
     // method returning. This means, for example, that you can't use pthread cancelation or signals to
     // cause early return.
     //
     // Past timeout: it's possible for waitUntil() to be called with a timeout in the past. In that
     // case, waitUntil() will still release the lock and reacquire it. waitUntil() will always return
     // false in that case. This is subtly different from some pthread_cond_timedwait() implementations,
     // which may not release the lock for past timeout. But, this behavior is consistent with OpenGroup
     // documentation for timedwait().
     template<typename LockType>
     bool waitUntil(LockType& lock, const TimeWithDynamicClockType& timeout)
     {
         bool result;
         if (timeout < timeout.nowWithSameClock()) {
             lock.unlock();
             result = false;
         } else {
             result = ParkingLot::parkConditionally(
                 &m_hasWaiters,
                 [this] () -> bool {
                     // Let everyone know that we will be waiting. Do this while we hold the queue lock,
                     // to prevent races with notifyOne().
                     m_hasWaiters.store(true);
                     return true;
                 },
                 [&lock] () { lock.unlock(); },
                 timeout).wasUnparked;
         }
         lock.lock();
         return result;
     }

     // Wait until the given predicate is satisfied. Returns true if it is satisfied in the end.
     // May return early due to timeout.
     template<typename LockType, typename Functor>
     bool waitUntil(
         LockType& lock, const TimeWithDynamicClockType& timeout, const Functor& predicate)
     {
         while (!predicate()) {
             if (!waitUntil(lock, timeout))
                 return predicate();
         }
         return true;
     }

     // Wait until the given predicate is satisfied. Returns true if it is satisfied in the end.
     // May return early due to timeout.
     template<typename LockType, typename Functor>
     bool waitFor(
         LockType& lock, Seconds relativeTimeout, const Functor& predicate)
     {
         return waitUntil(lock, MonotonicTime::now() + relativeTimeout, predicate);
     }

     template<typename LockType>
     bool waitFor(LockType& lock, Seconds relativeTimeout)
     {
         return waitUntil(lock, MonotonicTime::now() + relativeTimeout);
     }

     template<typename LockType>
     void wait(LockType& lock)
     {
         waitUntil(lock, Time::infinity());
     }

     template<typename LockType, typename Functor>
     void wait(LockType& lock, const Functor& predicate)
     {
         while (!predicate())
             wait(lock);
     }

     // Note that this method is extremely fast when nobody is waiting. It is not necessary to try to
     // avoid calling this method. This returns true if someone was actually woken up.
     bool notifyOne()
     {
         if (!m_hasWaiters.load()) {
             // At this exact instant, there is nobody waiting on this condition. The way to visualize
             // this is that if unparkOne() ran to completion without obstructions at this moment, it
             // wouldn't wake anyone up. Hence, we have nothing to do!
             return false;
         }

         bool didNotifyThread = false;
         ParkingLot::unparkOne(
             &m_hasWaiters,
             [&] (ParkingLot::UnparkResult result) -> intptr_t {
                 if (!result.mayHaveMoreThreads)
                     m_hasWaiters.store(false);
                 didNotifyThread = result.didUnparkThread;
                 return 0;
             });
         return didNotifyThread;
     }

     void notifyAll()
     {
         if (!m_hasWaiters.load()) {
             // See above.
             return;
         }

         // It's totally safe for us to set this to false without any locking, because this thread is
         // guaranteed to then unparkAll() anyway. So, if there is a race with some thread calling
         // wait() just before this store happens, that thread is guaranteed to be awoken by the call to
         // unparkAll(), below.
         m_hasWaiters.store(false);

         ParkingLot::unparkAll(&m_hasWaiters);
     }

 private:
     Atomic<bool> m_hasWaiters { false };
 };

 using StaticCondition = Condition;

 } // namespace WTF

 using WTF::Condition;
 using WTF::StaticCondition;

 #endif // WTF_Condition_h
	/*
	* Copyright (C) 2015-2016 Apple Inc. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
	* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
	* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
	* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
	* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
	* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
	* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#ifndef WTF_Condition_h
	#define WTF_Condition_h

	#include <wtf/Noncopyable.h>
	#include <wtf/ParkingLot.h>
	#include <wtf/TimeWithDynamicClockType.h>

	namespace WTF {

	// This is a condition variable that is suitable for use with any lock-like object, including
	// our own WTF::Lock. It features standard wait()/notifyOne()/notifyAll() methods in addition to
	// a variety of wait-with-timeout methods. This includes methods that use WTF's own notion of
	// time, like wall-clock time (i.e. WallTime) and monotonic time (i.e. MonotonicTime). This is
	// a very efficient condition variable. It only requires one byte of memory. notifyOne() and
	// notifyAll() require just a load and branch for the fast case where no thread is waiting.
	// This condition variable, when used with WTF::Lock, can outperform a system condition variable
	// and lock by up to 58x.
	class Condition {
	WTF_MAKE_NONCOPYABLE(Condition);
	public:
	// Condition will accept any kind of time and convert it internally, but this typedef tells
	// you what kind of time Condition would be able to use without conversions. However, if you
	// are unlikely to be affected by the cost of conversions, it is better to use MonotonicTime.
	using Time = ParkingLot::Time;

	constexpr Condition() = default;

	// Wait on a parking queue while releasing the given lock. It will unlock the lock just before
	// parking, and relock it upon wakeup. Returns true if we woke up due to some call to
	// notifyOne() or notifyAll(). Returns false if we woke up due to a timeout. Note that this form
	// of waitUntil() has some quirks:
	//
	// No spurious wake-up: in order for this to return before the timeout, some notifyOne() or
	// notifyAll() call must have happened. No scenario other than timeout or notify can lead to this
	// method returning. This means, for example, that you can't use pthread cancelation or signals to
	// cause early return.
	//
	// Past timeout: it's possible for waitUntil() to be called with a timeout in the past. In that
	// case, waitUntil() will still release the lock and reacquire it. waitUntil() will always return
	// false in that case. This is subtly different from some pthread_cond_timedwait() implementations,
	// which may not release the lock for past timeout. But, this behavior is consistent with OpenGroup
	// documentation for timedwait().
	template<typename LockType>
	bool waitUntil(LockType& lock, const TimeWithDynamicClockType& timeout)
	{
	bool result;
	if (timeout < timeout.nowWithSameClock()) {
	lock.unlock();
	result = false;
	} else {
	result = ParkingLot::parkConditionally(
	&m_hasWaiters,
	[this] () -> bool {
	// Let everyone know that we will be waiting. Do this while we hold the queue lock,
	// to prevent races with notifyOne().
	m_hasWaiters.store(true);
	return true;
	},
	[&lock] () { lock.unlock(); },
	timeout).wasUnparked;
	}
	lock.lock();
	return result;
	}

	// Wait until the given predicate is satisfied. Returns true if it is satisfied in the end.
	// May return early due to timeout.
	template<typename LockType, typename Functor>
	bool waitUntil(
	LockType& lock, const TimeWithDynamicClockType& timeout, const Functor& predicate)
	{
	while (!predicate()) {
	if (!waitUntil(lock, timeout))
	return predicate();
	}
	return true;
	}

	// Wait until the given predicate is satisfied. Returns true if it is satisfied in the end.
	// May return early due to timeout.
	template<typename LockType, typename Functor>
	bool waitFor(
	LockType& lock, Seconds relativeTimeout, const Functor& predicate)
	{
	return waitUntil(lock, MonotonicTime::now() + relativeTimeout, predicate);
	}

	template<typename LockType>
	bool waitFor(LockType& lock, Seconds relativeTimeout)
	{
	return waitUntil(lock, MonotonicTime::now() + relativeTimeout);
	}

	template<typename LockType>
	void wait(LockType& lock)
	{
	waitUntil(lock, Time::infinity());
	}

	template<typename LockType, typename Functor>
	void wait(LockType& lock, const Functor& predicate)
	{
	while (!predicate())
	wait(lock);
	}

	// Note that this method is extremely fast when nobody is waiting. It is not necessary to try to
	// avoid calling this method. This returns true if someone was actually woken up.
	bool notifyOne()
	{
	if (!m_hasWaiters.load()) {
	// At this exact instant, there is nobody waiting on this condition. The way to visualize
	// this is that if unparkOne() ran to completion without obstructions at this moment, it
	// wouldn't wake anyone up. Hence, we have nothing to do!
	return false;
	}

	bool didNotifyThread = false;
	ParkingLot::unparkOne(
	&m_hasWaiters,
	[&] (ParkingLot::UnparkResult result) -> intptr_t {
	if (!result.mayHaveMoreThreads)
	m_hasWaiters.store(false);
	didNotifyThread = result.didUnparkThread;
	return 0;
	});
	return didNotifyThread;
	}

	void notifyAll()
	{
	if (!m_hasWaiters.load()) {
	// See above.
	return;
	}

	// It's totally safe for us to set this to false without any locking, because this thread is
	// guaranteed to then unparkAll() anyway. So, if there is a race with some thread calling
	// wait() just before this store happens, that thread is guaranteed to be awoken by the call to
	// unparkAll(), below.
	m_hasWaiters.store(false);

	ParkingLot::unparkAll(&m_hasWaiters);
	}

	private:
	Atomic<bool> m_hasWaiters { false };
	};

	using StaticCondition = Condition;

	} // namespace WTF

	using WTF::Condition;
	using WTF::StaticCondition;

	#endif // WTF_Condition_h