[PATCH v4 00/15] Add futex2 syscalls

[Andrey Semashev]

On 6/8/21 2:13 PM, Greg KH wrote:
> On Tue, Jun 08, 2021 at 02:03:50PM +0300, Andrey Semashev wrote:
>> On 6/8/21 4:25 AM, Nicholas Piggin wrote:
>>>
>>> Are shared pthread mutexes using existing pthread APIs that are today
>>> implemented okay with futex1 system call a good reason to constrain
>>> futex2 I wonder? Or do we have an opportunity to make a bigger change
>>> to the API so it suffers less from non deterministic latency (for
>>> example)?
>>
>> If futex2 is not able to cover futex1 use cases then it cannot be viewed as
>> a replacement. In the long term this means futex1 cannot be deprecated and
>> has to be maintained. My impression was that futex1 was basically
>> unmaintainable(*) and futex2 was an evolution of futex1 so that users of
>> futex1 could migrate relatively easily and futex1 eventually removed. Maybe
>> my impression was wrong, but I would like to see futex2 as a replacement and
>> extension of futex1, so the latter can be deprecated at some point.
> 
> You can never delete a kernel system call, so even if you "deprecate"
> it, it still needs to be supported for forever.

If I'm not mistaken, some syscalls were dropped from kernel in the past, 
after it was established they are no longer used. So it is not 
impossible, though might be more difficult specifically with futex.

> Best of all would be if internally your "futex2" code would replace the
> "futex1" code so that there is no two different code bases.  That would
> be the only sane way forward, having 2 code bases to work with is just
> insane.

Yes, implementing futex1 in terms of futex2 internally is a possible way 
forward. Though I'm not sure it is reasonable to require that to be done 
in the initial futex2 submission. This requires all of the futex1 
functionality to implemented in futex2 from the start, which I think is 
too much to ask. Even with some futex1 features missing, futex2 would be 
already very much useful to users, and it is easier to implement the 
missing bits incrementally over time.

Also, one other point I'd like to make is that not all futex1 features 
might need to be reimplemented if futex2 provides a better alternative. 
For example, as a user, I would like to see a different approach to 
robust futexes that does not mandate a single user (libc) and allows to 
use robust futexes directly.

>> (*) I use "unmaintainable" in a broad sense here. It exists and works in
>> newer kernel versions and may receive code changes that are necessary to
>> keep it working, but maintainers refuse any extensions or modifications of
>> the code, mostly because of its complexity.
> 
> Adding additional complexity for no good reason is not a good idea,
> especially if you are asking others to maintain and support that
> complexity.  Would you want to have to do that work?
> 
> So what's keeping the futex2 code from doing all that futex1 does so
> that the futex1 code can be deleted internally?

I think, André will answer this, but my guess is, as stated above, this 
is a lot of work and time while the intermediate version is already useful.