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

Florian Weimer
2019-11-05T14:28:13Z

* 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 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

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=19004>

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