Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking ABIs at the same time.

Thomas Gleixner
2019-11-05T11:56:46Z

On Tue, 5 Nov 2019, Florian Weimer wrote:
> * Thomas Gleixner:
> > On Tue, 5 Nov 2019, Florian Weimer wrote:
> >> * Shawn Landden:
> >> > If this new ABI is used, then bit 1 of the *next pointer of the
> >> > user-space robust_list indicates that the futex_offset2 value should
> >> > be used in place of the existing futex_offset.
> >>
> >> The futex interface currently has some races which can only be fixed by
> >> API changes. I'm concerned that we sacrifice the last bit for some
> >> rather obscure feature. What if we need that bit for fixing the
> >> correctness issues?
> >
> > That current approach is going nowhere and if we change the ABI ever then
> > this needs to happen with all *libc folks involved and agreeing.
> >
> > Out of curiosity, what's the race issue vs. robust list which you are
> > trying to solve?
>
> Sadly I'm not trying to solve them. Here's one of the issues:
>
> <https://sourceware.org/bugzilla/show_bug.cgi?id=14485>

That one seems more a life time problem, i.e. the mutex is destroyed,
memory freed and map address reused while another thread was not yet out of
the mutex_unlock() call. Nasty.

Thanks,

tglx