spinMutex.h

//
// Copyright 2023 Pixar
//
// Licensed under the terms set forth in the LICENSE.txt file available at
// https://openusd.org/license.
//
#ifndef PXR_BASE_TF_SPIN_MUTEX_H
#define PXR_BASE_TF_SPIN_MUTEX_H
 
#include "pxr/pxr.h"
#include "pxr/base/tf/api.h"
 
#include "pxr/base/arch/hints.h"
#include "pxr/base/tf/diagnosticLite.h"
 
#include <atomic>
#include <utility>
 
PXR_NAMESPACE_OPEN_SCOPE
 
class TfSpinMutex
{
public:
 
    TfSpinMutex() : _lockState(false) {}
 
    struct DeferAcquire {};
 
    static constexpr DeferAcquire deferAcquire {};
 
    struct ScopedLock {
 
        explicit ScopedLock(TfSpinMutex &m) : _mutex(&m) {
            Acquire();
        }
 
        ScopedLock(TfSpinMutex &m, TfSpinMutex::DeferAcquire) : _mutex(&m) {}
 
        ScopedLock() = default;
 
        ScopedLock(ScopedLock &&other) noexcept
            : _mutex(std::exchange(other._mutex, nullptr))
            , _acquired(std::exchange(other._acquired, false)) {}
 
        ScopedLock &operator=(ScopedLock &&other) noexcept {
            if (this != &other) {
                Release();
                _mutex = std::exchange(other._mutex, nullptr);
                _acquired = std::exchange(other._acquired, false);
            }
            return *this;
        }        
 
        ~ScopedLock() {
            Release();
        }
 
        void Acquire(TfSpinMutex &m) {
            Release();
            _mutex = &m;
            Acquire();
        }
 
        void Release() {
            if (_acquired) {
                _Release();
            }
        }
 
        void Acquire() {
            TF_DEV_AXIOM(_mutex);
            TF_DEV_AXIOM(!_acquired);
            _mutex->Acquire();
            _acquired = true;
        }
 
        bool TryAcquire(TfSpinMutex &m) {
            Release();
            _mutex = &m;
            return TryAcquire();
        }
 
        bool TryAcquire() {
            TF_DEV_AXIOM(_mutex);
            TF_DEV_AXIOM(!_acquired);
            _acquired = _mutex->TryAcquire();
            return _acquired;
        }
        
    private:
 
        void _Release() {
            TF_DEV_AXIOM(_acquired);
            _mutex->Release();
            _acquired = false;
        }
 
        TfSpinMutex *_mutex = nullptr;
        bool _acquired = false;
    };
 
    inline bool TryAcquire() {
        return _lockState.exchange(true, std::memory_order_acquire) == false;
    }
 
    void Acquire() {
        if (ARCH_LIKELY(TryAcquire())) {
            return;
        }
        _AcquireContended();
    }
 
    inline void Release() {
        _lockState.store(false, std::memory_order_release);
    }
 
private:
    
    TF_API void _AcquireContended();
    
    std::atomic<bool> _lockState;
};
 
PXR_NAMESPACE_CLOSE_SCOPE
 
#endif // PXR_BASE_TF_SPIN_MUTEX_H