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

Boqun Feng
2025-06-19T15:00:39Z

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