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

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Date: Mon, 25 Mar 2024 10:44:45 +0000
From: Mark Rutland <mark.rutland@....com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, llvm@...ts.linux.dev,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Bj"orn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,
	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,
	Luc Maranget <luc.maranget@...ia.fr>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Akira Yokosawa <akiyks@...il.com>,
	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, elver@...gle.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,
	torvalds@...ux-foundation.org, linux-arm-kernel@...ts.infradead.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

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.