Re: [WIP 0/3] Memory model and atomic API in Rust

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Mar 22, 2024 at 07:57:41PM -0400, Kent Overstreet wrote:
> On Fri, Mar 22, 2024 at 04:38:35PM -0700, Boqun Feng wrote:
> > Hi,
> > 
> > Since I see more and more Rust code is comming in, I feel like this
> > should be sent sooner rather than later, so here is a WIP to open the
> > discussion and get feedback.
> > 
> > One of the most important questions we need to answer is: which
> > memory (ordering) model we should use when developing Rust in Linux
> > kernel, given Rust has its own memory ordering model[1]. I had some
> > discussion with Rust language community to understand their position
> > on this:
> > 
> > 	https://github.com/rust-lang/unsafe-code-guidelines/issues/348#issuecomment-1218407557
> > 	https://github.com/rust-lang/unsafe-code-guidelines/issues/476#issue-2001382992
> > 
> > My takeaway from these discussions, along with other offline discussion
> > is that supporting two memory models is challenging for both correctness
> > reasoning (some one needs to provide a model) and implementation (one
> > model needs to be aware of the other model). So that's not wise to do
> > (at least at the beginning). So the most reasonable option to me is:
> > 
> > 	we only use LKMM for Rust code in kernel (i.e. avoid using
> > 	Rust's own atomic).
> > 
> > Because kernel developers are more familiar with LKMM and when Rust code
> > interacts with C code, it has to use the model that C code uses.
> 
> I wonder about that. The disadvantage of only supporting LKMM atomics is
> that we'll be incompatible with third party code, and we don't want to
> be rolling all of our own data structures forever.
> 

A possible solution to that is a set of C++ memory model atomics
implemented by LKMM atomics. That should be possible.

> Do we see a path towards eventually supporting the standard Rust model?
> 

Things that Rust/C++ memory model don't suppor but we use are at least:
mixed size atomics (cmpxchg a u64, but read a u8 from another thread),
dependencies (we used a lot in fast path), so it's not trivial.

There are also issues like where one Rust thread does a store(..,
RELEASE), and a C thread does a rcu_deference(), in practice, it
probably works but no one works out (and no one would work out) a model
to describe such an interaction.

Regards,
Boqun

> Perhaps LKMM atomics could be reworked to be a layer on top of C/C++
> atomics. When I last looked, they didn't look completely incompatible;
> rather, there is a common subset that both support with the same
> semantics, and either has some things that it supports and the other
> doesn't (i.e., LKMLL atomics have smp_mb__after_atomic(); this is just a
> straightforward optimization to avoid an unnecessary barrier on
> architectures where the atomic already provided it).