From: Jens Gustedt <jens.gustedt@inria.fr>
To: musl@lists.openwall.com
Subject: Re: Explaining cond var destroy [Re: [musl] C threads, v3.0]
Date: Wed, 06 Aug 2014 22:55:10 +0200 [thread overview]
Message-ID: <1407358510.24324.328.camel@eris.loria.fr> (raw)
In-Reply-To: <20140806173254.GC1674@brightrain.aerifal.cx>
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Am Mittwoch, den 06.08.2014, 13:32 -0400 schrieb Rich Felker:
> On Wed, Aug 06, 2014 at 06:56:56PM +0200, Jens Gustedt wrote:
> > Easy perhaps not. But the futex syscall returns some of the
> > information that is currently thrown away.
>
> Sadly it seems like a bad idea to rely on any of the information
> returned by the futex syscall, due to the possibility of spurious
> wakes coming from reuse of memory. This is discussed somewhat on glibc
> issue 13690:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=13690
that is a complicated thread, I am not sure that I captured all
aspects
> The core issue is that, when performing operations like a mutex
> unlock, we can only know if futex wake is needed _after_ the atomic
> release action has been performed, and at that point, the address on
> which the futex wake is performed may no longer be valid, or may have
> been reallocated for another use. Thus, futex wait for a new
> synchronization object can spuriously return 0.
*If* we suppose that your destroy-wait trick works (and I think it
does) such a reuse for pthread_cond_t can never happen, and thus the
futex-syscall should return the correct values. I'll try to work some
simplification from there.
> I think trying to preclude the possibility of such spurious wakes
> would require a lot more unlock/destroy synchronization than what we
> have now in cond vars.
Perhaps one could just let some spurious wakeups in some weird
situations escape to user space.
We'll see.
Jens
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next prev parent reply other threads:[~2014-08-06 20:55 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-08-04 9:30 C threads, v3.0 Jens Gustedt
2014-08-04 9:33 ` Jens Gustedt
2014-08-04 14:50 ` Rich Felker
2014-08-04 16:48 ` Jens Gustedt
2014-08-04 17:06 ` Rich Felker
2014-08-04 22:16 ` Jens Gustedt
2014-08-04 22:36 ` Rich Felker
2014-08-06 3:52 ` Explaining cond var destroy [Re: [musl] C threads, v3.0] Rich Felker
2014-08-06 8:43 ` Jens Gustedt
2014-08-06 9:41 ` Jens Gustedt
2014-08-06 10:03 ` Rich Felker
2014-08-06 10:32 ` Jens Gustedt
2014-08-06 16:15 ` Rich Felker
2014-08-06 16:56 ` Jens Gustedt
2014-08-06 17:32 ` Rich Felker
2014-08-06 20:55 ` Jens Gustedt [this message]
2014-08-06 22:04 ` Rich Felker
2014-08-06 22:43 ` Jens Gustedt
2014-08-06 23:15 ` Rich Felker
2014-08-07 7:50 ` Jens Gustedt
2014-08-07 10:52 ` Szabolcs Nagy
2014-08-07 11:03 ` Jens Gustedt
2014-08-07 16:13 ` Rich Felker
2014-08-07 16:47 ` Jens Gustedt
2014-08-07 17:25 ` Rich Felker
2014-08-08 9:20 ` Jens Gustedt
2014-08-08 16:53 ` Rich Felker
2014-08-08 19:14 ` Rich Felker
2014-08-08 20:48 ` Rich Felker
2014-08-09 6:47 ` Jens Gustedt
2014-08-12 2:50 ` Rich Felker
2014-08-12 7:04 ` Jens Gustedt
2014-08-12 16:01 ` Rich Felker
2014-08-12 19:09 ` Jens Gustedt
2014-08-12 21:18 ` Rich Felker
2014-08-13 6:43 ` Jens Gustedt
2014-08-13 7:19 ` Jens Gustedt
2014-08-06 9:50 ` Rich Felker
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