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).
