linux-kernel - Re: [PATCH v5 03/10] rust: sync: atomic: Add ordering annotation types

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Message-ID: <20250619143214.GJ1613376@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 16:32:14 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	lkmm@...ts.linux.dev, linux-arch@...r.kernel.org,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, Will Deacon <will@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Lyude Paul <lyude@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Mitchell Levy <levymitchell0@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5 03/10] rust: sync: atomic: Add ordering annotation
 types

On Thu, Jun 19, 2025 at 06:29:29AM -0700, Boqun Feng wrote:
> On Thu, Jun 19, 2025 at 12:31:55PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 18, 2025 at 09:49:27AM -0700, Boqun Feng wrote:
> > 
> > > +//! Memory orderings.
> > > +//!
> > > +//! The semantics of these orderings follows the [`LKMM`] definitions and rules.
> > > +//!
> > > +//! - [`Acquire`] and [`Release`] are similar to their counterpart in Rust memory model.
> > 
> > So I've no clue what the Rust memory model states, and I'm assuming
> > it is very similar to the C11 model. I have also forgotten what LKMM
> > states :/
> > 
> > Do they all agree on what RELEASE+ACQUIRE makes?
> > 
> 
> I think the question is irrelevant here, because we are implementing
> LKMM atomics in Rust using primitives from C, so no C11/Rust memory
> model in the picture for kernel Rust.

The question is relevant in so far that the comment refers to them; and
if their behaviour is different in any way, this is confusing.

> But I think they do. I assume you mostly ask whether RELEASE(a) +
> ACQUIRE(b) (i.e. release and acquire on different variables) makes a TSO
> barrier [1]? We don't make it a TSO barrier in LKMM either (only
> unlock(a)+lock(b) is a TSO barrier) and neither does C11/Rust memory
> model.
> 
> [1]: https://lore.kernel.org/lkml/20211202005053.3131071-1-paulmck@kernel.org/

Right, that!

So given we build locks from atomics, this has to come from somewhere.

The simplest lock -- TAS -- is: rmw.acquire + store.release.

So while plain store.release + load.acquire might not make TSO (although
IIRC ARM added variants that do just that in an effort to aid x86
emulation); store.release + rmw.acquire must, otherwise we cannot
satisfy that unlock+lock.