linux-kernel - Re: linux-next: manual merge of the rseq tree with Linus' tree

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Message-ID: <952535108.15867.1510760492893.JavaMail.zimbra@efficios.com>
Date:   Wed, 15 Nov 2017 15:41:32 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: linux-next: manual merge of the rseq tree with Linus' tree

----- On Nov 15, 2017, at 10:22 AM, Thomas Gleixner tglx@...utronix.de wrote:

> On Wed, 15 Nov 2017, Mathieu Desnoyers wrote:
>> ----- On Nov 15, 2017, at 9:48 AM, Stephen Rothwell sfr@...b.auug.org.au wrote:
>> 
>> > Hi Ingo,
>> > 
>> > On Wed, 15 Nov 2017 09:07:12 +0100 Ingo Molnar <mingo@...nel.org> wrote:
>> >>
>> >> There's absolutely no way such invasive x86 changes should be done outside the
>> >> x86
>> >> tree and be merged into linux-next.
>> >> 
>> >> linux-next should be for the regular maintenance flow, for changes pushed by
>> >> maintainers and part of the regular maintenance process - not for
>> >> work-in-progress
>> >> features that may or may not be merged upstream in that form ...
>> > 
>> > Sure.  I was given the impression that Linus was going to be asked to
>> > merge this tree during this merge window, so I assumed that it had been
>> > seen by the appropriate people.  Most of these patches include you,
>> > Linus and Andrew (among others) on their cc's and they seem to have
>> > gone through several revisions.
>> > 
>> > I guess Mathieu has jumped the gun.
>> 
>> The membarrier core serializing command has been developed in the open
>> since Aug. 27, 2017 [1]. That's one full development cycle. On Sept 28,
>> 2017, I started CCing Ingo when I noticed I would have to add documentation
>> to each architecture return-to-usermode paths [2]. I have never heard feedback
>> from him until now.
>> 
>> I am open to remove the x86-specific membarrier core serializing patch from
>> my tree and hand it to the x86 maintainers after the generic code makes it
>> into Linus' tree, if this is seen as the appropriate way forward.
> 
> To be honest, I was looking at this stuff losely, but the continous flux of
> it and the entanglement with the rseq series did not really help.
> 
> I have no idea why you are trying to force shove that rseq stuff into
> 4.15. It's not been complete before the merge window opened and it's still
> not complete AFAICT.

The rseq and cpu_opv parts have not moved much in the past weeks, and the
core of their development was done publicly since Oct. 12, 2017 [1]. It has
been discussed at the Kernel Summit as well.

[1] lkml.kernel.org/r/20171012230326.19984-1-mathieu.desnoyers@...icios.com

> 
> That x86 membarrier thing is not yet clear if it is required at all, so why
> is this so high priority?

I think you refer to the migration "bug" wrt core serialization, which is
a different topic. The membarrier sync_core command allows JIT to ensure
a core serializing instruction is issued on every core before they return
to that mm, before the JIT reclaims code memory. Those are different topics.

> Can we please handle this as any other feature which is not ready before
> the merge window and postpone the rseq stuff for 4.16?

Linus showed interest in merging the rseq tree when we discussed at the
Kernel Summit. The tree has not changed much since then. I only integrated
membarrier patches that were implemented around the time of the 4.14 merge
window, which I queued for the 4.15 (current) window.

> The core serialization thing might be a correctness issue and we can fix
> that independently once we have gathered enough information from
> Intel/AMD.

Even though the membarrier sync_core command will be relevant independently
of Intel and AMD's feedback, I'm open to postpone this x86-specific patch.

Thanks,

Mathieu

> 
> Thanks,
> 
> 	tglx

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com