c++ - Is this non-blocking counter implementation correct? - Stack Overflow

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Let's pretend I'm writing code for some imaginary CPU which supports plain atomic load and store operations, but has no support for CAS or RMW. Can this non-blocking counter implementation be considered correct?

namespace magic {

template <typename T>
T atomic_load(T & location); // atomically reads value from memory

template <typename T>
void atomic_store(T & location, T value) // atomically stores value in memory

} // namespace magic

class Counter {
public:
    int fetch_add(int val) {
        // assume all threads are pre-registered
        int & this_thread_request = _requests.at(std::this_thread::get_id());

        // if this thread takes the lock, it publishes all the modifications it has
        if (_mutex.try_lock()) {
            int old = magic::atomic_load(_val) + this_thread_request;
            magic::atomic_store(_val, old + val);
            this_thread_request = 0;
            _mutex.unlock();
            return old;
        }

        // if the lock is already taken, modification is saved locally and published later
        int old = magic::atomic_load(_val) + this_thread_request;
        this_thread_request = this_thread_request + val;
        return old;
    }

private:
    std::unordered_map<std::thread::id, int> _requests;

    std::mutex _mutex;
    int _val = 0;
};

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