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

Greg KH
2021-06-08T12:34:27Z

On Tue, Jun 08, 2021 at 03:06:48PM +0300, Andrey Semashev wrote:
> 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.

Those syscalls were all "compatible with other obsolete operating
system" syscalls from what I remember. You can still run binaries
built in 1995 just fine on your system today (I have a few around here
somewhere...)

Thinking that you can drop futex() in the next 10+ years is very wishful
thinking given just how slow userspace applications are ever updated,
sorry.

greg k-h