Chapter 7. queue_mpmc.cpp

[Ilya21]

Hello subjum. I have tried to execution your realization of lock-free queue_mpmc, but i recevied segmenation fault (core dump) in push function. Can you help me with this problem, please?

[LinRuiiXin]

@Ilya21

Hello subjum. I have tried to execution your realization of lock-free queue_mpmc, but i recevied segmenation fault (core dump) in push function. Can you help me with this problem, please?

I encountered the same issue, and I assume you’ve resolved it. Let me supplement the solution to this problem:

Memory Alignment Issue with counted_node_ptr Structure (May Occur in Certain Environments):
Differences in compilers might cause the compare_exchange_strong call on std::counted_node_ptr to fail, even when the expected value matches the actual value. To address this, modify the counted_node_ptr structure as follows:

struct counted_node_ptr {
    int     external_count;
    node*   ptr;
    uint8_t padding[sizeof(void*) - sizeof(int)];
};

Then modify the push function:

void push(T new_value) {
    std::unique_ptr<T> new_data(new T(new_value));
    counted_node_ptr new_next;
    new_next.ptr = new node;
    new_next.external_count = 1;
    counted_node_ptr old_tail = tail.load();
    for (;;) {
        increase_external_count(tail, old_tail);
        T* old_data = nullptr;
        if (old_tail.ptr->data.compare_exchange_strong(old_data, new_data.get())) {
            counted_node_ptr old_next = {0, nullptr, {0}};
            if (!old_tail.ptr->next.compare_exchange_strong(old_next, new_next)) { 
                delete new_next.ptr;
                new_next = old_next;
            }
            set_new_tail(old_tail, new_next);
            new_data.release();
            break;
        } else {
            counted_node_ptr old_next = {0, nullptr, {0}};
            if (old_tail.ptr->next.compare_exchange_strong(old_next, new_next)) {
                old_next = new_next;
                new_next.ptr = new node;
            }
            set_new_tail(old_tail, old_next);
        }
    }
}

Also, modify the node constructor:

node() {
    data = nullptr;
    node_counter new_count;
    new_count.internal_count = 0;
    new_count.external_counters = 2;
    count.store(new_count);
    counted_node_ptr new_next {0, nullptr, {0}};
    next.store(new_next);
}

Improper Queue Initialization:
This is the primary cause of the error. Modify the node constructor of the lock_free_queue as follows:

lock_free_queue() {
    counted_node_ptr dummy {1, new node, {0}};
    head = dummy;
    tail = dummy;
}

That’s all.