lab-threads-3

tasks:
  - rewrite queue-simulation.c to use RW locks
    - add another thread that will print the number of items
      in the queue less frequently than the main thread
  - run atomic-add in multiple modes, compare the results
  - write pthread_barrier impl
    - start with phtread_barrier_t, pthread_barrier_wait()
  - rewrite semaphores/posix-sem-fixed-race.c to use unnamed
    semaphores. Are they kernel persistent ?