Date: Tue, 12 Aug 2014 17:18:37 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Explaining cond var destroy [Re: C threads, v3.0] 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. 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. Rich
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