Date: Wed, 13 Aug 2014 09:19:05 +0200 From: Jens Gustedt <jens.gustedt@...ia.fr> To: musl@...ts.openwall.com Subject: Re: Explaining cond var destroy [Re: C threads, v3.0] Am Mittwoch, den 13.08.2014, 08:43 +0200 schrieb Jens Gustedt: > Am Dienstag, den 12.08.2014, 17:18 -0400 schrieb Rich Felker: > > On Tue, Aug 12, 2014 at 09:09:21PM +0200, Jens Gustedt wrote: > > > Rich, > > > thanks a lot for looking into the code. > > > > > > Am Dienstag, den 12.08.2014, 12:01 -0400 schrieb Rich Felker: > > > > As far as I can tell, the only thing that's saving you from sending > > > > futex wakes after free is that you're just using spinlocks. > > > > > > No, I don't think so. These protect critical sections at the beginning > > > of the cnd_t calls. The cnd_*wait calls hold the mutex at that time > > > anyhow, so even if these would be implemented with mutexes (an extra > > > one per cnd_t to protect the critical section) this wouldn't cause > > > late wakes, I think. > > > > I was talking about the unref-and-free code that's using spinlocks. > > I don't understand, the unref-and-free code doesn't use spinlocks. It > just uses an atomic_fetch_sub of 1 to determine if it is the last > user. If it is not, it does nothing and returns. > > (This is indeed supposing that atomic_fetch_sub is "lock-free" in the > sense of the C standard, which basically means that it has no > observable state from a signal handler. All architectures I know of > have that property, but my knowledge is limited.) ah, perhaps you meant that the unref-and-free was done while holding the spinlock? Maybe in the version that I posted this was the case. That was a bug that I have now corrected. I also have added an initial check for the wakers to see if they have anything to do before they go into the spinlock. This should basically solve the herd-doing-wakeup problem, since as soon as all waiters are considered to be unblocked, wakers will return immediately without spinning. Jens -- :: INRIA Nancy Grand Est ::: AlGorille ::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: Download attachment "signature.asc" of type "application/pgp-signature" (199 bytes)
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