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

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Message-ID: <aFQmDoRSEmUuPIQG@Mac.home>
Date: Thu, 19 Jun 2025 08:00:30 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
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 04:32:14PM +0200, Peter Zijlstra wrote:
> 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.
> 

I did use the word "similar" and before that I said "The semantics of
these orderings follows the [`LKMM`] definitions and rules." The
referring was merely to avoid repeating the part like:

- [`Acquire`] orders the load part of the operation against all
  following memory operations.

- [`Release`] orders the store part of the operation against all
  preceding memory operations.
  
because of this part, both models agree. But if you think this way is
better, I could change it.

> > 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.
> 

Make sense, so something like this in the model should work:

diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index d7e7bf13c831..90cb6db6e335 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -27,7 +27,7 @@ include "lock.cat"
 (* Release Acquire *)
 let acq-po = [Acquire] ; po ; [M]
 let po-rel = [M] ; po ; [Release]
-let po-unlock-lock-po = po ; [UL] ; (po|rf) ; [LKR] ; po
+let po-unlock-lock-po = po ; (([UL] ; (po|rf) ; [LKR]) | ([Release]; (po;rf); [Acquire & RMW])) ; po

 (* Fences *)
 let R4rmb = R \ Noreturn       (* Reads for which rmb works *)


although I'm not sure whether there will be actual users that use this
ordering.

Regards,
Boqun