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: <87sgn2skm6.fsf@oldenburg2.str.redhat.com>
Date:   Tue, 05 Nov 2019 11:06:25 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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.

* 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>

I think there are others, but I can't find a reference to them.  If
anyone wants to work on this, I can dig out the details and ask some
folks who have looked at this in the past.

Thanks,
Florian