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

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Message-ID: <alpine.DEB.2.21.1911051253430.17054@nanos.tec.linutronix.de>
Date:   Tue, 5 Nov 2019 12:56:40 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Florian Weimer <fweimer@...hat.com>
cc:     Shawn Landden <shawn@....icu>, libc-alpha@...rceware.org,
        linux-api@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Keith Packard <keithp@...thp.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC v2 PATCH] futex: extend set_robust_list to allow 2 locking
 ABIs at the same time.

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