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queueworld's Issues

Race condition in QwSpscUnorderedResultQueue::push()

QwSpscUnorderedResultQueue::push() currently uses the following algorithm:

void push(Node *node)
{
    Node *top =  atomicLifoTop_.load(std::memory_order_relaxed); // (*)
    node->next = top;
    atomicLifoTop_.store(node, std::memory_order_release);
}

The bug is that somewhere after (*) and prior to atomicLifoTop_.store(), the pop() function may execute:

node_ptr_type result = atomicLifoTop_.exchange(0, std::memory_order_acquire);

This would leave atomicLifoTop_->next pointing to a node that is no longer on the stack.

The fix is to use compare_exchange instead of store, to handle the above condition as a special case. (This fix will work because QwSpscUnorderedResultQueue is single-producer single-consumer, so it is not possible for other threads to interfere.)

void push(Node *node)
{
    Node *top =  atomicLifoTop_.load(std::memory_order_relaxed);
    node->next = top;

    if (atomicLifoTop_.compare_exchange_strong(top, node,
                /*success:*/ std::memory_order_release,
                /*failure:*/ std::memory_order_release) == false) { 
        assert(top == 0);
        node->next = 0;
        atomicLifoTop_.store(node, std::memory_order_relaxed);
    }
}

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