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

Nick Desaulniers
2017-11-16T18:16:25Z

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.

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.

>> > 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.

> 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.