Re: Resuming work on new semaphore

[Alexander Monakov <amonakov@ispras.ru>]

On Wed, 1 Apr 2015, Rich Felker wrote:
> If just waiting, the negative semaphore value persists after the
> waiter is killed. Subsequent posts will produce a wake for a waiter
> that doesn't exist, and will thereby allow future waiters that arrive
> when the semaphore value is zero to proceed immediately (leaving the
> value negative) by consuming this wake. There are usage patterns where
> trywait would never succeed again, but wait would succeed trivially.

Interesting.  To examine the issue under a different light, consider that from
the perspective of semaphore implementation, waiters that were killed,
stopped, or pre-empted forever in the middle of sem_wait are
indistinguishable.  Thus, subsequent sem_wait succeeds by effectively stealing
a post, and to make things consistent you can teach sem_trywait to steal posts
too (i.e. try atomic-decrement-if-positive val[1] just before returning
EAGAIN, return 0 if that succeeds).

Alexander