std::atomic<std::atomic<bool>*> lock;

    // Code that uses lock

    std::atomic<bool> my_flag = false;
    auto* other_flag = lock.exchange(&my_flag);

    // spin   
    if(other_flag) while(!other_flag->load());

    // You have the lock. Do work

    // Release lock

    auto* next_flag = &my_flag;
    if(!lock.compare_exchange_strong(next_flag, nullptr)) next_flag->store(true);