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Date: Wed, 06 Aug 2014 22:55:10 +0200
From: Jens Gustedt <>
Subject: Re: Explaining cond var destroy [Re: C threads, v3.0]

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:

that is a complicated thread, I am not sure that I captured all

> 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.


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