Date: Thu, 2 Apr 2015 10:42:16 +0300 (MSK) From: Alexander Monakov <amonakov@...ras.ru> To: musl@...ts.openwall.com Subject: Re: Resuming work on new semaphore 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 just before returning EAGAIN, return 0 if that succeeds). Alexander
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