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/*
* 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.
*/
#pragma once
#include <wtf/LockAlgorithm.h>
#include <wtf/Locker.h>
#include <wtf/Noncopyable.h>
namespace TestWebKitAPI {
struct LockInspector;
}
namespace WTF {
typedef LockAlgorithm<uint8_t, 1, 2> DefaultLockAlgorithm;
// This is a fully adaptive mutex that only requires 1 byte of storage. It has fast paths that are
// competetive to a spinlock (uncontended locking is inlined and is just a CAS, microcontention is
// handled by spinning and yielding), and a slow path that is competetive to std::mutex (if a lock
// cannot be acquired in a short period of time, the thread is put to sleep until the lock is
// available again). It uses less memory than a std::mutex. This lock guarantees eventual stochastic
// fairness, even in programs that relock the lock immediately after unlocking it. Except when there
// are collisions between this lock and other locks in the ParkingLot, this lock will guarantee that
// at worst one call to unlock() per millisecond will do a direct hand-off to the thread that is at
// the head of the queue. When there are collisions, each collision increases the fair unlock delay
// by one millisecond in the worst case.
class Lock {
WTF_MAKE_NONCOPYABLE(Lock);
WTF_MAKE_FAST_ALLOCATED;
public:
constexpr Lock() = default;
void lock()
{
if (UNLIKELY(!DefaultLockAlgorithm::lockFastAssumingZero(m_byte)))
lockSlow();
}
bool tryLock()
{
return DefaultLockAlgorithm::tryLock(m_byte);
}
// Need this version for std::unique_lock.
bool try_lock()
{
return tryLock();
}
// Relinquish the lock. Either one of the threads that were waiting for the lock, or some other
// thread that happens to be running, will be able to grab the lock. This bit of unfairness is
// called barging, and we allow it because it maximizes throughput. However, we bound how unfair
// barging can get by ensuring that every once in a while, when there is a thread waiting on the
// lock, we hand the lock to that thread directly. Every time unlock() finds a thread waiting,
// we check if the last time that we did a fair unlock was more than roughly 1ms ago; if so, we
// unlock fairly. Fairness matters most for long critical sections, and this virtually
// guarantees that long critical sections always get a fair lock.
void unlock()
{
if (UNLIKELY(!DefaultLockAlgorithm::unlockFastAssumingZero(m_byte)))
unlockSlow();
}
// This is like unlock() but it guarantees that we unlock the lock fairly. For short critical
// sections, this is much slower than unlock(). For long critical sections, unlock() will learn
// to be fair anyway. However, if you plan to relock the lock right after unlocking and you want
// to ensure that some other thread runs in the meantime, this is probably the function you
// want.
void unlockFairly()
{
if (UNLIKELY(!DefaultLockAlgorithm::unlockFastAssumingZero(m_byte)))
unlockFairlySlow();
}
void safepoint()
{
if (UNLIKELY(!DefaultLockAlgorithm::safepointFast(m_byte)))
safepointSlow();
}
bool isHeld() const
{
return DefaultLockAlgorithm::isLocked(m_byte);
}
bool isLocked() const
{
return isHeld();
}
private:
friend struct TestWebKitAPI::LockInspector;
static const uint8_t isHeldBit = 1;
static const uint8_t hasParkedBit = 2;
WTF_EXPORT_PRIVATE void lockSlow();
WTF_EXPORT_PRIVATE void unlockSlow();
WTF_EXPORT_PRIVATE void unlockFairlySlow();
WTF_EXPORT_PRIVATE void safepointSlow();
// Method used for testing only.
bool isFullyReset() const
{
return !m_byte.load();
}
Atomic<uint8_t> m_byte { 0 };
};
using LockHolder = Locker<Lock>;
} // namespace WTF
using WTF::Lock;
using WTF::LockHolder;