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/*
* Copyright (C) 2015-2019 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/Atomics.h>
#include <wtf/Compiler.h>
namespace WTF {
// This is the algorithm used by WTF::Lock. You can use it to project one lock onto any atomic
// field. The limit of one lock is due to the use of the field's address as a key to find the lock's
// queue.
template<typename LockType>
struct EmptyLockHooks {
static LockType lockHook(LockType value) { return value; }
static LockType unlockHook(LockType value) { return value; }
static LockType parkHook(LockType value) { return value; }
static LockType handoffHook(LockType value) { return value; }
};
template<typename LockType, LockType isHeldBit, LockType hasParkedBit, typename Hooks = EmptyLockHooks<LockType>>
class LockAlgorithm {
static constexpr bool verbose = false;
static constexpr LockType mask = isHeldBit | hasParkedBit;
public:
static bool lockFastAssumingZero(Atomic<LockType>& lock)
{
return lock.compareExchangeWeak(0, Hooks::lockHook(isHeldBit), std::memory_order_acquire);
}
static bool lockFast(Atomic<LockType>& lock)
{
return lock.transaction(
[&] (LockType& value) -> bool {
if (value & isHeldBit)
return false;
value |= isHeldBit;
value = Hooks::lockHook(value);
return true;
},
std::memory_order_acquire);
}
static void lock(Atomic<LockType>& lock)
{
if (UNLIKELY(!lockFast(lock)))
lockSlow(lock);
}
static bool tryLock(Atomic<LockType>& lock)
{
for (;;) {
LockType currentValue = lock.load(std::memory_order_relaxed);
if (currentValue & isHeldBit)
return false;
if (lock.compareExchangeWeak(currentValue, Hooks::lockHook(currentValue | isHeldBit), std::memory_order_acquire))
return true;
}
}
static bool unlockFastAssumingZero(Atomic<LockType>& lock)
{
return lock.compareExchangeWeak(isHeldBit, Hooks::unlockHook(0), std::memory_order_release);
}
static bool unlockFast(Atomic<LockType>& lock)
{
return lock.transaction(
[&] (LockType& value) -> bool {
if ((value & mask) != isHeldBit)
return false;
value &= ~isHeldBit;
value = Hooks::unlockHook(value);
return true;
},
std::memory_order_release);
}
static void unlock(Atomic<LockType>& lock)
{
if (UNLIKELY(!unlockFast(lock)))
unlockSlow(lock, Unfair);
}
static void unlockFairly(Atomic<LockType>& lock)
{
if (UNLIKELY(!unlockFast(lock)))
unlockSlow(lock, Fair);
}
static bool safepointFast(const Atomic<LockType>& lock)
{
WTF::compilerFence();
return !(lock.load(std::memory_order_relaxed) & hasParkedBit);
}
static void safepoint(Atomic<LockType>& lock)
{
if (UNLIKELY(!safepointFast(lock)))
safepointSlow(lock);
}
static bool isLocked(const Atomic<LockType>& lock)
{
return lock.load(std::memory_order_acquire) & isHeldBit;
}
NEVER_INLINE static void lockSlow(Atomic<LockType>& lock);
enum Fairness {
Unfair,
Fair
};
NEVER_INLINE static void unlockSlow(Atomic<LockType>& lock, Fairness fairness = Unfair);
NEVER_INLINE static void safepointSlow(Atomic<LockType>& lockWord)
{
unlockFairly(lockWord);
lock(lockWord);
}
private:
enum Token {
BargingOpportunity,
DirectHandoff
};
};
} // namespace WTF
using WTF::LockAlgorithm;
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