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Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking ABIs at the same time.
- From: Thomas Gleixner <tglx at linutronix dot de>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Shawn Landden <shawn at git dot icu>, libc-alpha at sourceware dot org, linux-api at vger dot kernel dot org, LKML <linux-kernel at vger dot kernel dot org>, Arnd Bergmann <arnd at arndb dot de>, Deepa Dinamani <deepa dot kernel at gmail dot com>, Oleg Nesterov <oleg at redhat dot com>, Andrew Morton <akpm at linux-foundation dot org>, Catalin Marinas <catalin dot marinas at arm dot com>, Keith Packard <keithp at keithp dot com>, Peter Zijlstra <peterz at infradead dot org>
- Date: Tue, 5 Nov 2019 12:56:40 +0100 (CET)
- Subject: Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking ABIs at the same time.
- References: <20191104002909.25783-1-shawn@git.icu> <87woceslfs.fsf@oldenburg2.str.redhat.com> <alpine.DEB.2.21.1911051053470.17054@nanos.tec.linutronix.de> <87sgn2skm6.fsf@oldenburg2.str.redhat.com>
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