[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <alpine.LNX.2.11.1504030021400.8195@monopod.intra.ispras.ru>
Date: Fri, 3 Apr 2015 00:39:10 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: Resuming work on new semaphore
On Thu, 2 Apr 2015, Rich Felker wrote:
> > 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.
Did you mean "presence"? I'm having trouble understanding your phrase,
especially after "assuming ..."; can you elaborate or rephrase?
That waiters can die breaks an assumption that operations on val[0] and val[1]
do not under/overflow due to their range exceeding the number of
simultaneously live tasks.
> > 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).
>
> 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[0])
> 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.
I think my proposition above needs at least the following correction: when
trywait succeeds in stealing a post by dec-if-positive(val[1]), it should also
decrement val[0] before returning.
Are you sure your proposition is invalid? I don't think so. How is trywait
different from a timedwait with a timeout that immediately expires? That is
basically what your scheme should do.
Alexander
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
