On Thu, Apr 15, 2021 at 11:04:37PM -0700, Nick Desaulniers wrote: > On Thu, Apr 15, 2021 at 9:27 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote: > > > > [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. > > Yes, I agree; I think that's a very good approach. Avoiding the ISO > WG14 is interesting; at least the merits could be debated in the > public and not behind closed doors. WG14 (C) and WG21 (C++) are at least somewhat open. Here are some of the proposals a few of us have in flight: P2055R0 A Relaxed Guide to memory_order_relaxed http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2055r0.pdf P0124R7 Linux-Kernel Memory Model (vs. that of C/C++) http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0124r7.html P1726R4 Pointer lifetime-end zap http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1726r4.pdf https://docs.google.com/document/d/1MfagxTa6H0rTxtq9Oxyh4X53NzKqOt7y3hZBVzO_LMk/edit?usp=sharing P1121R2 Hazard Pointers: Proposed Interface and Wording for Concurrency TS 2 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1121r2.pdf P1382R1 volatile_load<T> and volatile_store<T> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1382r1.pdf P1122R2 Proposed Wording for Concurrent Data Structures: Read-Copy-Update (RCU) http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1122r2.pdf https://docs.google.com/document/d/1MfagxTa6H0rTxtq9Oxyh4X53NzKqOt7y3hZBVzO_LMk/edit?usp=sharing P0190R4 Proposal for New memory order consume Definition http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0190r4.pdf P0750R1 Consume http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0750r1.html P1726R4 is of particular concern, along with consume. > > 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. I believe that compatibility with both C/C++ and the Linux kernel are important. Thanx, Paul
