c++ synchronization with conditional variable

I am trying to synchronize the increment of the variable i alternatively with main thread and thread t. Below programme is blocked after few iterations. I couldn't figure out what the problem is.

#include <iostream>
#include <thread>
#include <algorithm>
#include <mutex>
#include <condition_variable>
using namespace std;
mutex mu;
condition_variable cv_main;
condition_variable cv_thr;
bool make_main_work;
bool make_thread_work;
void fun(int& i)
{
    unique_lock<mutex> ul(mu);
    cv_thr.wait(ul, [](){  return make_thread_work;});
    cout << "fun: " <<++i<<endl;
    make_thread_work = false;

    ul.unlock();
    make_main_work = true;
    cv_main.notify_one();
}

int main()
{
    int i =0;
    make_thread_work = false;
    make_main_work = true;

    thread t(fun, std::ref(i));
    for(int k =1; k<100; ++k)
    {
    unique_lock<mutex> ul(mu);
    cv_main.wait(ul, [](){return make_main_work;});
    make_main_work = false;
    cout << "main: "<<++i << endl;
    ul.unlock();
    make_thread_work = true;
    cv_thr.notify_one();
    }

    return 0;
}