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From:   Florian Weimer <fweimer@redhat.com>
To:     Carlos O'Donell <carlos@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Shawn Landden <shawn@git.icu>, libc-alpha@sourceware.org,
        linux-api@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Deepa Dinamani <deepa.kernel@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Keith Packard <keithp@keithp.com>,
        Peter Zijlstra <peterz@infradead.org>
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>
        <alpine.DEB.2.21.1911051253430.17054@nanos.tec.linutronix.de>
        <f11d82f1-1e81-e344-3ad2-76e4cb488a3d@redhat.com>
Date:   Tue, 05 Nov 2019 15:27:48 +0100
In-Reply-To: <f11d82f1-1e81-e344-3ad2-76e4cb488a3d@redhat.com> (Carlos
        O'Donell's message of "Tue, 5 Nov 2019 09:10:38 -0500")
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* Carlos O'Donell:

> On 11/5/19 6:56 AM, Thomas Gleixner wrote:
>> 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 t=
hen
>>>> 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=3D14485>
>>=20
>> 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.
>
> It is difficult to fix.
>
> The other issue is this:
>
> "Robust mutexes do not take ROBUST_LIST_LIMIT into account"
> https://sourceware.org/bugzilla/show_bug.cgi?id=3D19089

That's just a missing check in our implementation and something that few
applications will encounter, if any.  There is this one here:

  <https://sourceware.org/bugzilla/show_bug.cgi?id=3D19004>

It contains a kernel patch.

I thought that there were more issues in the current implementation, but
I can't a record of them. 8-(

Thanks,
Florian