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

Florian Weimer
2019-11-05T10:06:41Z

* 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