Date: Thu, 2 Apr 2015 11:26:43 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Resuming work on new semaphore On Thu, Apr 02, 2015 at 10:42:16AM +0300, Alexander Monakov wrote: > 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. Yes, I noticed this too. In that sense, theoretically there should be no harm (aside from eventual overflow of pending wake counter) from having asynchronously-killed waiters, assuming the implementation is bug-free in the absence of async killing of waiters. > 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). Hmm, perhaps that is valid. I'll have to think about it again. I was thinking of having sem_trywait unconditionally down the value (val) then immitate the exit path of sem_timedwait, but that's not valid because another waiter could race and prevent sem_trywait from ever being able to exit. But if it only does the down as a dec-if-positive then it seems like it can safely dec-if-positive the wake count before reporting failure. Rich
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