Re: Resuming work on new semaphore

[Alexander Monakov <amonakov@ispras.ru>]

On Sun, 5 Apr 2015, Rich Felker wrote:
> 1. Thread A enters sem_wait.
> 2. Thread B observes thread A in sem_wait via failed sem_trywait.

Hm, I don't see how that can be achieved.  As a result I'm afraid I didn't
fully understand your example.

> > Well we can make sem_getvalue return val[0]+val[1] instead... ;)
> 
> That just makes the new implementation look like the old one, no? :-)

Can't be bad if it behaves the same but works a bit faster.
Apropos, like I've said on IRC, looks like there's "semaphore uncertainty
principle": that formal semaphore value is between val[0] and (val[0] +/-
val[1]) (clamped to 0 as needed).  It seems you can either do your hack and
pretend that there are never any waiters, or try to faithfully count waiters
in sem_getvalue, but then also reveal that sometimes the implementation works
by stealing a post.  I believe you could argue that the latter is explicitely
disallowed by the spec.

By the way, I think there's an interesting interplay with cancellation.
Consider the following.  Thread B does "return sem_wait(sem);". Thread A does:

  pthread_cancel(thread_B);
  sem_post(sem);
  sem_getvalue(sem);

If it observes semaphore value as 1 it follows that thread B has not become a
waiter yet, and since it must have cancellation already pending, it may not
consume the post.  And yet if thread B is already futex-waiting in sem_wait,
consuming the post takes priority over acting on cancellation.  So if then
thread A does

  pthread_join(thread_B);
  sem_getvalue(sem);

and gets value of 0, it sees a contradiction.  And return value from
pthread_join will indicate that thread_B exited normally rather than was
cancelled.

And on the contrary, if you make acting on cancellation/timeout take priority,
you can observe semaphore value increasing when waiters leave the wait on
error path without consuming the post.

Alexander