Re: [PATCH v4 00/15] Add futex2 syscalls

Andrey Semashev
2021-06-08T11:03:59Z

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.

In any case, creating a new API should consider requirements of its
potential users. If futex2 is intended to eventually replace futex1 then
all current futex1 users are potential users of futex2. If not, then the
futex2 submission should list its intended users, at least in general
terms, and their requirements that led to the proposed API design.

(*) 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.