LockHelper.hpp

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/*
 *  Copyright 2019-2021 Diligent Graphics LLC
 *  Copyright 2015-2019 Egor Yusov
 *  
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *  
 *      http://www.apache.org/licenses/LICENSE-2.0
 *  
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *  In no event and under no legal theory, whether in tort (including negligence), 
 *  contract, or otherwise, unless required by applicable law (such as deliberate 
 *  and grossly negligent acts) or agreed to in writing, shall any Contributor be
 *  liable for any damages, including any direct, indirect, special, incidental, 
 *  or consequential damages of any character arising as a result of this License or 
 *  out of the use or inability to use the software (including but not limited to damages 
 *  for loss of goodwill, work stoppage, computer failure or malfunction, or any and 
 *  all other commercial damages or losses), even if such Contributor has been advised 
 *  of the possibility of such damages.
 */
 
#pragma once
 
#include "../../Platforms/interface/Atomics.hpp"
#include "../../Platforms/Basic/interface/DebugUtilities.hpp"
 
namespace ThreadingTools
{
 
class LockFlag
{
public:
    enum
    {
        LOCK_FLAG_UNLOCKED = 0,
        LOCK_FLAG_LOCKED   = 1
    };
    LockFlag(Atomics::Long InitFlag = LOCK_FLAG_UNLOCKED) noexcept
    {
        //m_Flag.store(InitFlag);
        m_Flag = InitFlag;
    }
 
    operator Atomics::Long() const { return m_Flag; }
 
private:
    friend class LockHelper;
    Atomics::AtomicLong m_Flag;
};
 
// Spinlock implementation. This kind of lock should be used in scenarios
// where simultaneous access is uncommon but possible.
class LockHelper
{
public:
    LockHelper() noexcept {}
 
    LockHelper(LockFlag& LockFlag) noexcept
    {
        Lock(LockFlag);
    }
 
    LockHelper(LockHelper&& LockHelper) noexcept :
        m_pLockFlag{std::move(LockHelper.m_pLockFlag)}
    {
        LockHelper.m_pLockFlag = nullptr;
    }
 
    const LockHelper& operator=(LockHelper&& LockHelper) noexcept
    {
        m_pLockFlag            = std::move(LockHelper.m_pLockFlag);
        LockHelper.m_pLockFlag = nullptr;
        return *this;
    }
 
    ~LockHelper()
    {
        Unlock();
    }
 
    static bool UnsafeTryLock(LockFlag& LockFlag) noexcept
    {
        return Atomics::AtomicCompareExchange(LockFlag.m_Flag,
                                              static_cast<Atomics::Long>(LockFlag::LOCK_FLAG_LOCKED),
                                              static_cast<Atomics::Long>(LockFlag::LOCK_FLAG_UNLOCKED)) == LockFlag::LOCK_FLAG_UNLOCKED;
    }
 
    bool TryLock(LockFlag& LockFlag) noexcept
    {
        if (UnsafeTryLock(LockFlag))
        {
            m_pLockFlag = &LockFlag;
            return true;
        }
        else
            return false;
    }
 
    static constexpr const int DefaultSpinCountToYield = 256;
 
    static void UnsafeLock(LockFlag& LockFlag, int SpinCountToYield = DefaultSpinCountToYield) noexcept
    {
        int SpinCount = 0;
        while (!UnsafeTryLock(LockFlag))
        {
            ++SpinCount;
            if (SpinCount == SpinCountToYield)
            {
                SpinCount = 0;
                YieldThread();
            }
        }
    }
 
    void Lock(LockFlag& LockFlag, int SpinCountToYield = DefaultSpinCountToYield) noexcept
    {
        VERIFY(m_pLockFlag == NULL, "Object already locked");
        // Wait for the flag to become unlocked and lock it
        int SpinCount = 0;
        while (!TryLock(LockFlag))
        {
            ++SpinCount;
            if (SpinCount == SpinCountToYield)
            {
                SpinCount = 0;
                YieldThread();
            }
        }
    }
 
    static void UnsafeUnlock(LockFlag& LockFlag) noexcept
    {
        LockFlag.m_Flag = LockFlag::LOCK_FLAG_UNLOCKED;
    }
 
    void Unlock() noexcept
    {
        if (m_pLockFlag)
            UnsafeUnlock(*m_pLockFlag);
        m_pLockFlag = NULL;
    }
 
private:
    static void YieldThread() noexcept;
 
    LockFlag* m_pLockFlag = nullptr;
 
    // clang-format off
    LockHelper           (const LockHelper& LockHelper) = delete;
    LockHelper& operator=(const LockHelper& LockHelper) = delete;
    // clang-format on
};
 
} // namespace ThreadingTools