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

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Date:   Tue, 5 Nov 2019 09:10:38 -0500
From:   Carlos O'Donell <carlos@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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 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 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.

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=19089

-- 
Cheers,
Carlos.