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

Peter Zijlstra
2025-06-19T14:32:22Z

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.