#ifndef WTF_LockAlgorithm_h
#define WTF_LockAlgorithm_h
#include <wtf/Atomics.h>
#include <wtf/Compiler.h>
namespace WTF {
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 const bool verbose = false;
static const 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_relaxed);
}
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
};
};
}
using WTF::LockAlgorithm;
#endif // WTF_LockAlgorithm_h