Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG

Paul E. McKenney
2017-11-16T18:40:00Z

On Thu, Nov 16, 2017 at 10:16:22AM -0800, Nick Desaulniers wrote:
> On Thu, Nov 16, 2017 at 9:48 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Thu, Nov 16, 2017 at 06:34:17PM +0100, Peter Zijlstra wrote:
> >> On Thu, Nov 16, 2017 at 09:16:49AM -0800, Nick Desaulniers wrote:
> >> > On Thu, Nov 16, 2017 at 8:59 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >> > > On Thu, Nov 16, 2017 at 08:50:41AM -0800, Nick Desaulniers wrote:
> >> > >> On Thu, Nov 16, 2017 at 8:30 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >> > >>
> >> > >> > Ideally we'd get the toolchain people to commit to supporting the kernel
> >> > >> > memory model along side the C11 one. That would help a ton.
> >> > >>
> >> > >> Does anyone from the kernel side participate in the C standardization process?
> >> > >
> >> > > Yes, Paul McKenney and Will Deacon. Doesn't mean these two can still be
> >> > > reconciled though. From what I understand C11 (and onwards) are
> >> > > incompatible with the kernel model on a number of subtle points.
> >> >
> >> > It would be good to have these incompatibilities written down, then
> >> > for the sake of argument, they can be cited both for discussions on
> >> > LKML and in the C standardization process. For example, a running
> >> > list in Documentation/ or something would make it so that anyone could
> >> > understand and cite current issues with the latest C standard.
> >>
> >> Will should be able to produce this list; I know he's done before, I
> >> just can't find it -- my Google-foo isn't strong today.
> >
> > Here you go:
> >
> > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0124r4.html
>
> Great, thanks! Will take some time to digest, but happy to refer
> others to this important work in the future.

Glad you like it!

> I wonder if we have anything like a case study that shows specifically
> how a compiler generated a subtle bug due to specifics of the memory
> model, like a "if you do this, here's the problematic code that will
> get generated, and why it's problematic due to the memory model."
> That may be a good way to raise issues with toolchain developers with
> concrete and actionable examples.

Well, the above is an official working paper from the C++ standards
committee.

The first priority is to fix memory_order_consume. Which is getting a
bit more mindshare of late. As Fedor Pikus said at CPPCON: "If you have
a large software project, you are probably already using RCU. But you
don't know that you are using it, and you are probably doing it wrong."

> >> > I don't understand why we'd block patches for enabling experimental
> >> > features. We've been running this patch-set on actual devices for
> >> > months and would love to provide them to the community for further
> >> > testing. If bugs are found, then there's more evidence to bring to
> >> > the C standards committee. Otherwise we're shutting down feature
> >> > development for the sake of potential bugs in a C standard we're not
> >> > even using.
> >>
> >> So the problem is that its very very hard (and painful) to find these
> >> bugs. Getting the tools people to comment on these specific
> >> optimizations would really help lots.
>
> No doubt; I do not disagree with you. Kernel developers have very
> important use cases for the language.
>
> But the core point I'm trying to make is "do we need to block this
> patch set until issues with the C standards body in regards to the
> kernels memory model are resolved?" I would hope the two are
> orthogonal and that we'd take them and then test them even more
> extensively than the developer has in order to find out.

Given that I have been working on getting the C and C++ standards to
correctly handle rcu_dereference() for more than ten years, I recommend
-against- waiting on standardization in the strongest possible terms.
And if you think that ten years is bad, the Java standards community has
been struggling with out-of-thin-air (OOTA) values for almost 20 years.
And the C and C++ standards haven't solved OOTA, either.

Thanx, Paul

> > It would be good to get something similar to LKMM into KTSAN, for
> > example. There would probably be a few differences due to efficiency
> > concerns, but closer is better than less close. ;-)
>
> + glider, who may be able to comment on the state of KTSAN.
>