C++ simple mutex using atomic_flag (code not working)

This is an exercise of using atomic_flag with acquire/release memory model to implement a very simple mutex. There are THREADS number of threads, and each thread increment cou LOOP number of times. The threads are synchronized with this simple mutex. However, the code throws exception in thread.join() function. Could someone please enlighten me why this does not work? Thank you in advance!

#include <atomic>
#include <thread>
#include <assert.h>
#include <vector>

using namespace std;
class mutex_simplified {
private:
    atomic_flag flag;
public:
    void lock() {
        while (flag.test_and_set(memory_order_acquire));
    }

    void unlock() {
        flag.clear(memory_order_release);
    }
};

mutex_simplified m_s;
int cou(0);
const int LOOP = 10000;
const int THREADS = 1000;
void increment() {
    for (unsigned i = 0; i < LOOP; i++) {
        m_s.lock();
        cou++;
        m_s.unlock();
    }
}

int main() {
    thread a(increment);
    thread b(increment);
    vector<thread> threads;

    for (int i = 0; i < THREADS; i++)
        threads.push_back(thread(increment));

    for (auto & t : threads) {
        t.join();
    }
    assert(cou == THREADS*LOOP);
}