使用std：：C++11无锁队列(多写入器，单用户)

Question

我使用新的std::atomic在C++11中生成了一个无锁(无锁)队列的简单实现，我看不出我在这里做了什么。

#include <atomic>

template<typename T>
class lockless_queue
{
public:
    template<typename DataType>
    struct node
    {
        node(const DataType& data)
          : data(data), next(nullptr) {}
        DataType data;
        node* next;
    };

    lockless_queue()
      : head_(nullptr) {}

    void produce(const T &data)
    {
        node<T>* new_node = new node<T>(data);
        // put the current value of head into new_node->next
        new_node->next = head_.load(std::memory_order_relaxed);
        // now make new_node the new head, but if the head
        // is no longer what's stored in new_node->next
        // (some other thread must have inserted a node just now)
        // then put that new head into new_node->next and try again
        while(!std::atomic_compare_exchange_weak_explicit(
            &head_,
            &new_node->next,
            new_node,
            std::memory_order_release,
            std::memory_order_relaxed)) {}
    }

    node<T>* consume_all()
    {
        // Reset queue and return head atomically
        return head_.exchange(nullptr, std::memory_order_consume);
    }
private:
    std::atomic<node<T>*> head_;
};

// main.cpp
#include <iostream>

int main()
{
    lockless_queue<int> s;
    s.produce(1);
    s.produce(2);
    s.produce(3);
    auto head = s.consume_all();
    while (head)
    {
        auto tmp = head->next;
        std::cout << tmp->data << std::endl;
        delete head;
        head = tmp;
    }
}

我的产出是：

2
1
Segmentation fault (core dumped)

我能有一些指点，看看哪里，或者指出我可能做错了什么？

谢谢!

Answer 1

您正在取消tmp而不是head的引用。

while (head)
    {
        auto tmp = head->next;
        std::cout << tmp->data << std::endl;
        delete head;
        head = tmp;
    }

应：

while (head)
    {
        std::cout << head->data << std::endl;
        auto tmp = head->next;
        delete head;
        head = tmp;
    }

这就是为什么3没有出现在您的输出中，而Segmentation fault却出现在输出中。

Answer 2

在您的代码中有另一个错误，除非您开始尝试执行并发的队列，否则不会出现。如果compare_exchange_weak_explicit失败，这意味着另一个线程设法更改了head指针，因此在您再次尝试head之前，需要将head指针的新值重新加载到new_node->next中。以下是其中的诀窍：

    while(!std::atomic_compare_exchange_weak_explicit(
        &head_,
        &new_node->next,
        new_node,
        std::memory_order_release,
        std::memory_order_relaxed)) {
        new_node->next = head_.load(std::memory_order_relaxed);
    }