Date: Wed, 13 Aug 2014 08:43:30 +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 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.) > If > it were using mutexes that don't protect against making futex wake > calls after the atomic unlock, a previous unref could send the wake > after the final one freed the object. So in effect, if you use a mutex > here, I think the wake-after-free issue has just been moved to a > different object, not solved. > > > > This is an > > > extremely expensive solution: While contention is rare, as soon as you > > > do hit contention, if there are many threads they all pile on and > > > start spinning, and the time to obtain a lock (and cpu time/energy > > > spent waiting) grows extremely high. And of course it becomes infinite > > > if you have any threads of differing priorities and the low-priority > > > thread has the lock... > > > > I think you dramatize a bit :) > > Perhaps. :) > > > It is very unlikely that a thread that reaches the critical section is > > unscheduled *during* that critical section. If it is unscheduled, you > > are right, the wait can be long. But that event is very unlikely, so > > the average time inside the critical section is still short, with a > > probability distribution that is a bit skewed because of the > > outliers. > > Yes. The general pathology of spinlocks is that they give extremely > high latency and cpu load in an extremely low probability worst-case. > > > (And then there is no concept of different scheduling priorities for C > > threads, all of them are equal.) > > Indeed, but there's no reason these functions couldn't end up getting > called from a POSIX program using a C11 library. This is the normal > expected usage for mutexes (i.e. you're writing a library that needs > to be thread-safe but you don't want to depend on POSIX -- in practice > the calling application is unlikely to be using C11 thrd_create > because it sucks :) and perhaps less likely but definitely not > impossible for cond vars. Hm, C threads are meant primarily as a portable and simple user space tool and as a means to provide a model of parallelism as far as the C standard is concerned. And that model is flat and has no hierarchy among threads. 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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