where is the UB?

#include <iostream>
#include <climits>


long fetch_add(long& gcount, long addend)
{
    long lcount = gcount;
    gcount += addend;
    return lcount;
}


int main()
{
    long m_count = LONG_MAX;
    std::cout << "m_count = " << m_count << "\n";

    // simulate three concurret readers
    fetch_add(m_count, -3);
    std::cout << "m_count = " << m_count << ", 3 readers\n";

    // now m_count = LONG_MAX - 3

    // simulate a writer. There can only be a single writer.
    long count = fetch_add(m_count, -LONG_MAX);
    std::cout << "m_count = " << m_count << ", 3 readers in write mode\n";

    if (count < LONG_MAX)
    {
        std::cout << "\nReaders Detected!\n";
        long readers = LONG_MAX - count;
        std::cout << "count = " << count << "\n";
        std::cout << "readers = " << readers << "\n";
    }

    return 0;
}