Date:   Tue, 8 Jun 2021 14:35:14 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Andrey Semashev <andrey.semashev@...il.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        André Almeida <andrealmeid@...labora.com>,
        acme@...nel.org, Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        corbet@....net, Davidlohr Bueso <dave@...olabs.net>,
        Darren Hart <dvhart@...radead.org>, fweimer@...hat.com,
        joel@...lfernandes.org, kernel@...labora.com,
        krisman@...labora.com, libc-alpha@...rceware.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, malteskarupke@...tmail.fm,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        pgriffais@...vesoftware.com, Peter Oskolkov <posk@...k.io>,
        Steven Rostedt <rostedt@...dmis.org>, shuah@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>, z.figura12@...il.com
Subject: Re: [PATCH v4 00/15] Add futex2 syscalls

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.
> 
> > 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.

Then do it the other way around, as Peter points out.

> > 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.

useful to who?  I still do not understand what users will be needing
this.  All I can tell is a single userspace program wants to use it, and
that is a fork from the real project it was based on and that the
maintainers have no plan to merge it back.

So who does need/want this?

thanks,

greg k-h

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