c++ - 来自 Boost.Atomic 示例的无等待队列崩溃

Question

我一直在寻找使用原子的无等待队列的 C++ 实现，并找到了 Boost.Atomic 示例:

template<typename T>
class waitfree_queue {
public:
  struct node {
    T data;
    node * next;
  };
  void push(const T &data)
  {
    node * n = new node;
    n->data = data;
    node * stale_head = head_.load(boost::memory_order_relaxed);
    do {
      n->next = stale_head;
    } while (!head_.compare_exchange_weak(stale_head, n, boost::memory_order_release));
  }

  node * pop_all(void)
  {
    T * last = pop_all_reverse(), * first = 0;
    while(last) {
      T * tmp = last;
      last = last->next;
      tmp->next = first;
      first = tmp;
    }
    return first;
  }

  waitfree_queue() : head_(0) {}

  // alternative interface if ordering is of no importance
  node * pop_all_reverse(void)
  {
    return head_.exchange(0, boost::memory_order_consume);
  }
private:
  boost::atomic<node *> head_;
};

int main() {
// pop elements
waitfree_queue<int>::node * x = q.pop_all()
while(x) {
  X * tmp = x;
  x = x->next;
  // process tmp->data, probably delete it afterwards
  delete tmp;
}

}

Example at boost official site

我已将 boost 替换为 std 并使用 MSVC 2012 进行编译。它崩溃并在控制台中显示下一条消息:

Assertion failed: _Order2 != memory_order_release, file c:\program files (x86)\m
icrosoft visual studio 11.0\vc\include\xxatomic, line 742

当我编译原始 boost 时，它运行时崩溃。

它是 Boost.Atomic 中的错误还是原子的 MSVC 实现中的错误？

Answer 1

它看起来像是 MSVC 实现中的错误。断言失败，因为 Order2 是 memory_order_release。但是，如前所述here (与 C++ 标准相同)(强调我的):

The 3-parameter overload is equivalent to the 4-parameter overload with success_order==order, and failure_order==order, except that if order is std::memory_order_acq_rel, then failure_order is std::memory_order_acquire, and if order is std::memory_order_release then failure_order is std::memory_order_relaxed.

换句话说，Order2 必须是 std::memory_order_relaxed 对于您的情况下的 4 参数重载，因为您通过了 memory_order_release 作为order。 MSVC 的实现不是这种情况，这是一个错误。如果可能，请将其报告为错误。