Acquire a read lock. Several threads can hold this typeof lock.
        It is exclusive with write locks.

def acquire_read(self):
        """
        Acquire a read lock. Several threads can hold this typeof lock.
        It is exclusive with write locks.
        """
        self.monitor.acquire()
        while self.rwlock < 0 or self.writers_waiting:
            self.readers_ok.wait()
        self.rwlock += 1
        self.monitor.release()