Re: [PATCH 00/13] [RFC] Rust support

[Boqun Feng]

[Copy LKMM people, Josh, Nick and Wedson]

On Thu, Apr 15, 2021 at 08:58:16PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 14, 2021 at 08:45:51PM +0200, ojeda@xxxxxxxxxx wrote:
> 
> > Rust is a systems programming language that brings several key
> > advantages over C in the context of the Linux kernel:
> > 
> >   - No undefined behavior in the safe subset (when unsafe code is
> >     sound), including memory safety and the absence of data races.
> 
> And yet I see not a single mention of the Rust Memory Model and how it
> aligns (or not) with the LKMM. The C11 memory model for example is a
> really poor fit for LKMM.
> 

I think Rust currently uses C11 memory model as per:

	https://doc.rust-lang.org/nomicon/atomics.html

, also I guess another reason that they pick C11 memory model is because
LLVM has the support by default.

But I think the Rust Community still wants to have a good memory model,
and they are open to any kind of suggestion and input. I think we (LKMM
people) should really get involved, because the recent discussion on
RISC-V's atomics shows that if we didn't people might get a "broken"
design because they thought C11 memory model is good enough:

	https://lore.kernel.org/lkml/YGyZPCxJYGOvqYZQ@boqun-archlinux/

And the benefits are mutual: a) Linux Kernel Memory Model (LKMM) is
defined by combining the requirements of developers and the behavior of
hardwares, it's pratical and can be a very good input for memory model
designing in Rust; b) Once Rust has a better memory model, the compiler
technologies whatever Rust compilers use to suppor the memory model can
be adopted to C compilers and we can get that part for free.

At least I personally is very intereted to help Rust on a complete and
pratical memory model ;-)

Josh, I think it's good if we can connect to the people working on Rust
memoryg model, I think the right person is Ralf Jung and the right place
is https://github.com/rust-lang/unsafe-code-guidelines, but you
cerntainly know better than me ;-) Or maybe we can use Rust-for-Linux or
linux-toolchains list to discuss.

[...]
> >   - Boqun Feng is working hard on the different options for
> >     threading abstractions and has reviewed most of the `sync` PRs.
> 
> Boqun, I know you're familiar with LKMM, can you please talk about how
> Rust does things and how it interacts?

As Wedson said in the other email, currently there is no code requiring
synchronization between C side and Rust side, so we are currently fine.
But in the longer term, we need to teach Rust memory model about the
"design patterns" used in Linux kernel for parallel programming.

What I have been doing so far is reviewing patches which have memory
orderings in Rust-for-Linux project, try to make sure we don't include
memory ordering bugs for the beginning.

Regards,
Boqun