Semaphore implementation with mutex/cv is in the class notes.

Everyone did pretty well on the scheduling problem.  The only real
issue was definition of "response time": "completion time" - "arrival
time".

Also, no preemptive interleaving (timeslicing) for plain STCF.

producer/consumer with semaphores:

produce/send:
empty->P
(lock; add message to mailbox; unlock)
message->V

consume/receive:
message->P
(lock; get message from mailbox; unlock)
empty->V

Deadlock scenario: two threads send/receive to each other with enough
messages to almost fill the mailboxes.  Another thread writes a few
messages to each mailbox, enough to fill them.  There are
interleavings that will deadlock and some that will not.

Coroutines are similar to threads but it is a different abstraction
because the interleavings are determined by the program, with no
scheduler.  No concurrency, no parallelism, no non-determinism, and
hence no need for scheduling control.