From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/5815 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: Explaining cond var destroy [Re: [musl] C threads, v3.0] Date: Wed, 13 Aug 2014 08:43:30 +0200 Message-ID: <1407912210.4951.15.camel@eris.loria.fr> References: <1407430024.24324.387.camel@eris.loria.fr> <20140807172514.GI1674@brightrain.aerifal.cx> <1407489627.24324.419.camel@eris.loria.fr> <20140808191405.GP1674@brightrain.aerifal.cx> <20140808204855.GQ1674@brightrain.aerifal.cx> <1407566854.4988.231.camel@eris.loria.fr> <20140812025013.GY1674@brightrain.aerifal.cx> <1407827079.15134.109.camel@eris.loria.fr> <20140812160103.GB1674@brightrain.aerifal.cx> <1407870561.15134.152.camel@eris.loria.fr> <20140812211837.GC12888@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg="pgp-sha1"; 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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. >=20 > 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. >=20 > > > This is an > > > extremely expensive solution: While contention is rare, as soon as yo= u > > > 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 infinit= e > > > if you have any threads of differing priorities and the low-priority > > > thread has the lock... > >=20 > > I think you dramatize a bit :) >=20 > Perhaps. :) >=20 > > 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. >=20 > Yes. The general pathology of spinlocks is that they give extremely > high latency and cpu load in an extremely low probability worst-case. >=20 > > (And then there is no concept of different scheduling priorities for C > > threads, all of them are equal.) >=20 > 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 --=20 :: 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 :: --=-hgCpHc43cA9E3eMY6dHy Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iEYEABECAAYFAlPrCRIACgkQD9PoadrVN+IT/gCeMH/FG8lscKP9zn4VlJ/vkMcW NXsAmwZCgewB6EsDK0brJxO/1QQFJFvn =SaYa -----END PGP SIGNATURE----- --=-hgCpHc43cA9E3eMY6dHy--