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Re: race condition destroying condition variables

From: Samuel Thibault
Subject: Re: race condition destroying condition variables
Date: Wed, 27 Dec 2017 20:31:45 +0100
User-agent: NeoMutt/20170113 (1.7.2)


Brent W. Baccala, on mar. 26 déc. 2017 23:06:13 -0500, wrote:
> Also, the Linux source code in nptl/ includes the following comment:
>   /* If there are waiters which have been already signalled or
>      broadcasted, but still are using the pthread_cond_t structure,
>      pthread_cond_destroy needs to wait for them.  */

Ok, so even if Posix explicitly says that it has undefined behavior,
since nptl behaves fine we should probably behave fine too.

> The problem that I've got now is that I've changed the size of the condition
> variables by adding an extra field,

Not a good idea, for the resaons you mention :)

> First, I'd like to build the packages using the same scripts used to build the
> Debian distribution packages.  Is this available somewhere?

Just run dpkg-buildpackage from the package source, or use pdebuild, or
use a wanna-build infrastructure. Nothing simple, at any rate.

> Next, how will modifying the memory layout of condition variables affect our
> packaging?  I figure that we'll have to bump libc0.3 to libc0.4, and that
> should pretty much do the trick, right?

Yes, but that's a *HUGE* transition, which is painful. Better use symbol
versioning, so that old non-recompiled programs use the old
implementation, and recompiled programs used the new.

> Finally, I'm wondering if we need to change this at all.  The "__data" field 
> in
> struct __pthread_cond seems unused, at least in the libpthreads directory.  Is
> there any use for this pointer?  Or can I use those bits?

AFAICT it was used for compatibility with libcthreads, which we don't
care any more, so I'd say feel free to use it, and avoid the transition


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