If two or more threads are using the same data area, link, etc, it should be protected by a semaphore or by some other technique, to ensure that anomalies do not occur. See: semaphores and volatile.

If a thread tries to do a read on a channel, the thread  wait until another thread does a write on it; at that moment, the transfer happens. If, during the time the thread is waiting for the transfer to take place, a third thread tries to do a read on the same channel, the effects are unpredictable.