igaztanaga@gmail.com wrote:
My point is that (http://en.wikipedia.org/wiki/Semaphore_(programming)):
P(Semaphore s) // Acquire Resource { wait until s > 0, then s := s-1; /* Testing and decrementing s must be atomic to avoid race conditions */ }
V(Semaphore s) // Release Resource { s := s+1; /* must be atomic */ }
P can only wake a single thread, because the semaphore count represents the free resource count. The original post said:
"the post() unlocks only one of the 4 producers"
and that's logical. If there are 4 waiting threads, you need 4 posts to unlock all.
The post operation that 'wakes up' is V, not P. If your api allows V(4) to release 4 resources atomically, then 4 consumers in P must be released (if they are waiting, and any left over accrued to allow future consumers not to block. Even if you say V(4) is not supported and actually V;V;V;V and these are atomic individually, then there is still a race between those four posts and any Ps actually getting to run. Either way, you MUST enable 4-off Ps to become ready. If the API allows to post multiple resources atomically, then you need to handle this.