无锁栈的实现

template<typename T>
class LockFreeStack
{
private:
    struct Node;
    
    struct CountedNode
    {
        int   externalCount = 0;
        Node* ptr           = nullptr;
    };
    
    struct Node
    {
        std::shared_ptr<T> data;
        std::atomic<int>   internalCount;
        CountedNode        next;
        
        Node(T const& data_):
            data(std::make_shared<T>(data_)),
            internalCount(0)
        {}
    };
    
    std::atomic<CountedNode> head;
    
    void increaseHeadCount(CountedNode& oldCounter)
    {
        CountedNode newCounter;
        do {
            newCounter = oldCounter;
            ++newCounter.externalCount;
        } while (!head.compare_exchange_strong(oldCounter, newCounter,
                                               std::memory_order_acquire,
                                               std::memory_order_relaxed));
        oldCounter.externalCount = newCounter.externalCount;
    }
public:
    ~LockFreeStack()
    {
        while(pop() != nullptr);
    }
    
    void push(T const& data)
    {
        CountedNode newNode;
        newNode.ptr = new Node(data);
        newNode.externalCount = 1;
        newNode.ptr->next = head.load(std::memory_order_relaxed);
        while(!head.compare_exchange_weak(newNode.ptr->next, newNode,
                                          std::memory_order_release,
                                          std::memory_order_relaxed));
    }
    
    std::shared_ptr<T> pop()
    {
        auto oldHead = head.load(std::memory_order_relaxed);
        for(;;){
            increaseHeadCount(oldHead);
            auto const nodePtr = oldHead.ptr;
            
            if (nodePtr == nullptr){
                return shared_ptr<T>();
            }
            
            if (head.compare_exchange_strong(oldHead, nodePtr->next,
                                             std::memory_order_relaxed)){
                std::shared_ptr<T> result;
                result.swap(nodePtr->data);
                int const increaseCount = oldHead.externalCount - 2;
                if (nodePtr->internalCount.fetch_add(increaseCount,
                                                     std::memory_order_release)
                    == -increaseCount){
                    delete nodePtr;
                }
                return result;
            }
            else if (nodePtr->internalCount.fetch_add(-1,
                                            std::memory_order_acquire) == 1){
                nodePtr->internalCount.load(std::memory_order_acquire);
                delete nodePtr;
            }
        }
    }
};

为了测试其正确性，我用了以下代码作为实验：

    LockFreeStack<int> stack;
    std::thread t1([&]
                   {
                       for (int i = 0; i < 100; ++i){
                           if(i % 2 == 0){
                               stack.push(i);
                           }
                           else{
                               auto const result = stack.pop();
                               if (result != nullptr){
                                   cout << *result << " ";
                               }
                           }
                       }
                   });
    std::thread t2([&]
                   {
                       for (int i = 100; i < 200; ++i){
                           stack.push(i);
                       }
                   });
    std::thread t3([&]
                   {
                       for (int i = 0; i < 199; ++i){
                           auto const result = stack.pop();
                           if (result != nullptr){
                               cout << *result << " ";
                           }
                       }
                   });
    t1.join();
    t2.join();
    t3.join();

结果输出很奇怪，很多好多上千的数，我一度认为是栈实现的问题，直到我用 printf 替换了 cout 之后……