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

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Date: Fri, 22 Mar 2024 17:15:08 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
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>,
	Björn 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,
	Mark Rutland <mark.rutland@....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 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).