std::atomic<bool> flag{false};

void lock()

{

while (flag.exchange(true, std::memory_order_relaxed));

std::atomic_thread_fence(std::memory_order_acquire);

}

void unlock()

{

std::atomic_thread_fence(std::memory_order_release);

flag.store(false, std::memory_order_relaxed);

}

mutex mtx_a;

std::thread t1([&](){

mtx_a.lock();

a += 1;

mtx_a.unlock();

});

std::thread t2([&](){

mtx_a.lock();

a += 2;

mtx_a.unlock();

});

t1.join();

t2.join();

std::cout << a << std::endl;

return 0;

}