209 lines
7.2 KiB
C++
209 lines
7.2 KiB
C++
/*
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* Copyright (C) 2015-2019 Apple Inc. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
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* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
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* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
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* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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#pragma once
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#include <mutex>
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#include <wtf/LockAlgorithm.h>
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#include <wtf/Locker.h>
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#include <wtf/Noncopyable.h>
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#include <wtf/Seconds.h>
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#include <wtf/ThreadSafetyAnalysis.h>
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namespace TestWebKitAPI {
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struct LockInspector;
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}
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namespace WTF {
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typedef LockAlgorithm<uint8_t, 1, 2> DefaultLockAlgorithm;
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// This is a fully adaptive mutex that only requires 1 byte of storage. It has fast paths that are
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// competetive to a spinlock (uncontended locking is inlined and is just a CAS, microcontention is
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// handled by spinning and yielding), and a slow path that is competetive to std::mutex (if a lock
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// cannot be acquired in a short period of time, the thread is put to sleep until the lock is
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// available again). It uses less memory than a std::mutex. This lock guarantees eventual stochastic
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// fairness, even in programs that relock the lock immediately after unlocking it. Except when there
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// are collisions between this lock and other locks in the ParkingLot, this lock will guarantee that
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// at worst one call to unlock() per millisecond will do a direct hand-off to the thread that is at
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// the head of the queue. When there are collisions, each collision increases the fair unlock delay
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// by one millisecond in the worst case.
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//
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// This lock type supports thread safety analysis.
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// To annotate a member variable or a global variable with thread ownership information,
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// use lock capability annotations defined in ThreadSafetyAnalysis.h.
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class WTF_CAPABILITY_LOCK Lock {
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WTF_MAKE_NONCOPYABLE(Lock);
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WTF_MAKE_FAST_ALLOCATED;
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public:
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constexpr Lock() = default;
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void lock() WTF_ACQUIRES_LOCK()
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{
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if (UNLIKELY(!DefaultLockAlgorithm::lockFastAssumingZero(m_byte)))
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lockSlow();
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}
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bool tryLock() WTF_ACQUIRES_LOCK_IF(true) // NOLINT: Intentional deviation to support std::scoped_lock.
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{
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return DefaultLockAlgorithm::tryLock(m_byte);
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}
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// Need this version for std::unique_lock.
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bool try_lock() WTF_ACQUIRES_LOCK_IF(true)
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{
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return tryLock();
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}
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WTF_EXPORT_PRIVATE bool tryLockWithTimeout(Seconds timeout) WTF_ACQUIRES_LOCK_IF(true);
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// Relinquish the lock. Either one of the threads that were waiting for the lock, or some other
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// thread that happens to be running, will be able to grab the lock. This bit of unfairness is
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// called barging, and we allow it because it maximizes throughput. However, we bound how unfair
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// barging can get by ensuring that every once in a while, when there is a thread waiting on the
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// lock, we hand the lock to that thread directly. Every time unlock() finds a thread waiting,
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// we check if the last time that we did a fair unlock was more than roughly 1ms ago; if so, we
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// unlock fairly. Fairness matters most for long critical sections, and this virtually
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// guarantees that long critical sections always get a fair lock.
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void unlock() WTF_RELEASES_LOCK()
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{
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if (UNLIKELY(!DefaultLockAlgorithm::unlockFastAssumingZero(m_byte)))
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unlockSlow();
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}
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// This is like unlock() but it guarantees that we unlock the lock fairly. For short critical
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// sections, this is much slower than unlock(). For long critical sections, unlock() will learn
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// to be fair anyway. However, if you plan to relock the lock right after unlocking and you want
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// to ensure that some other thread runs in the meantime, this is probably the function you
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// want.
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void unlockFairly() WTF_RELEASES_LOCK()
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{
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if (UNLIKELY(!DefaultLockAlgorithm::unlockFastAssumingZero(m_byte)))
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unlockFairlySlow();
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}
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void safepoint()
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{
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if (UNLIKELY(!DefaultLockAlgorithm::safepointFast(m_byte)))
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safepointSlow();
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}
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bool isHeld() const
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{
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return DefaultLockAlgorithm::isLocked(m_byte);
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}
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bool isLocked() const
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{
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return isHeld();
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}
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private:
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friend struct TestWebKitAPI::LockInspector;
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static constexpr uint8_t isHeldBit = 1;
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static constexpr uint8_t hasParkedBit = 2;
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WTF_EXPORT_PRIVATE void lockSlow();
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WTF_EXPORT_PRIVATE void unlockSlow();
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WTF_EXPORT_PRIVATE void unlockFairlySlow();
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WTF_EXPORT_PRIVATE void safepointSlow();
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// Method used for testing only.
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bool isFullyReset() const
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{
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return !m_byte.load();
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}
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Atomic<uint8_t> m_byte { 0 };
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};
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// Asserts that the lock is held.
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// This can be used in cases where the annotations cannot be added to the function
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// declaration.
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inline void assertIsHeld(const Lock& lock) WTF_ASSERTS_ACQUIRED_LOCK(lock) { ASSERT_UNUSED(lock, lock.isHeld()); }
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// Locker specialization to use with Lock.
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// Non-movable simple scoped lock holder.
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// Example: Locker locker { m_lock };
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template <>
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class WTF_CAPABILITY_SCOPED_LOCK Locker<Lock> : public AbstractLocker {
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public:
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explicit Locker(Lock& lock) WTF_ACQUIRES_LOCK(lock)
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: m_lock(lock)
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, m_isLocked(true)
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{
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m_lock.lock();
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}
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Locker(AdoptLockTag, Lock& lock) WTF_REQUIRES_LOCK(lock)
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: m_lock(lock)
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, m_isLocked(true)
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{
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}
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~Locker() WTF_RELEASES_LOCK()
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{
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if (m_isLocked)
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m_lock.unlock();
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}
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void unlockEarly() WTF_RELEASES_LOCK()
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{
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ASSERT(m_isLocked);
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m_isLocked = false;
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m_lock.unlock();
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}
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Locker(const Locker<Lock>&) = delete;
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Locker& operator=(const Locker<Lock>&) = delete;
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private:
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// Support DropLockForScope even though it doesn't support thread safety analysis.
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template<typename>
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friend class DropLockForScope;
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void lock() WTF_ACQUIRES_LOCK(m_lock)
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{
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m_lock.lock();
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compilerFence();
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}
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void unlock() WTF_RELEASES_LOCK(m_lock)
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{
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compilerFence();
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m_lock.unlock();
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}
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Lock& m_lock;
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bool m_isLocked { false };
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};
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Locker(Lock&) -> Locker<Lock>;
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Locker(AdoptLockTag, Lock&) -> Locker<Lock>;
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using LockHolder = Locker<Lock>;
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} // namespace WTF
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using WTF::Lock;
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using WTF::LockHolder;
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