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 think that makes sense; if nothing else it's consistent with how we handle the C atomics today. > And this patchset is the result of that option. I introduced an atomic > library to wrap and implement LKMM atomics (of course, given it's a WIP, > so it's unfinished). Things to notice: > > * I know I could use Rust macro to generate the whole set of atomics, > but I choose not to in the beginning, as I want to make it easier to > review. > > * Very likely, we will only have AtomicI32, AtomicI64 and AtomicUsize > (i.e no atomic for bool, u8, u16, etc), with limited support for > atomic load and store on 8/16 bits. > > * I choose to re-implement atomics in Rust `asm` because we are still > figuring out how we can make it easy and maintainable for Rust to call > a C function _inlinely_ (Gary makes some progress [2]). Otherwise, > atomic primitives would be function calls, and that can be performance > bottleneck in a few cases. I don't think we want to maintain two copies of each architecture's atomics. This gets painful very quickly (e.g. as arm64's atomics get patched between LL/SC and LSE forms). Can we start off with out-of-line atomics, and see where the bottlenecks are? It's relatively easy to do that today, at least for the atomic*_*() APIs: https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=atomics/outlined&id=e0a77bfa63e7416d610769aa4ab62bc06993ce56 ... which IIUC covers the "AtomicI32, AtomicI64 and AtomicUsize" cases you mention above. Mark.
