NVIDIA Senior Linux Kernel Engineer interview questions

HR set up the phone screen, A guy called me and used collabedit to ask me question about how to read a value from a running counter. I told him to use mutex to run the two threads mutually exclusive : one thread for updating(write) the counter and one thread for reading the counter. But he had no idea on how the mutex works. After the phone screen, I also emailed him a sample codes. He never replied.