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

Boqun Feng
2025-06-19T13:29:33Z

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.

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/

> > +//! - [`Full`] means "fully-ordered", that is:
> > +//! - It provides ordering between all the preceding memory accesses and the annotated operation.
> > +//! - It provides ordering between the annotated operation and all the following memory accesses.
> > +//! - It provides ordering between all the preceding memory accesses and all the fllowing memory
> > +//! accesses.
> > +//! - All the orderings are the same strong as a full memory barrier (i.e. `smp_mb()`).
>
> s/strong/strength/ ?
>

Fixed locally.

Regards,
Boqun

> > +//! - [`Relaxed`] is similar to the counterpart in Rust memory model, except that dependency
> > +//! orderings are also honored in [`LKMM`]. Dependency orderings are described in "DEPENDENCY
> > +//! RELATIONS" in [`LKMM`]'s [`explanation`].
> > +//!
> > +//! [`LKMM`]: srctree/tools/memory-model/
> > +//! [`explanation`]: srctree/tools/memory-model/Documentation/explanation.txt