Re: [PATCH v7 3/9] rust: sync: atomic: Add ordering annotation types

["Benno Lossin" <lossin@kernel.org>]

On Mon Jul 14, 2025 at 7:36 AM CEST, Boqun Feng wrote:
> Preparation for atomic primitives. Instead of a suffix like _acquire, a
> method parameter along with the corresponding generic parameter will be
> used to specify the ordering of an atomic operations. For example,
> atomic load() can be defined as:
>
> 	impl<T: ...> Atomic<T> {
> 	    pub fn load<O: AcquireOrRelaxed>(&self, _o: O) -> T { ... }
> 	}
>
> and acquire users would do:
>
> 	let r = x.load(Acquire);
>
> relaxed users:
>
> 	let r = x.load(Relaxed);
>
> doing the following:
>
> 	let r = x.load(Release);
>
> will cause a compiler error.
>
> Compared to suffixes, it's easier to tell what ordering variants an
> operation has, and it also make it easier to unify the implementation of
> all ordering variants in one method via generic. The `TYPE` associate
> const is for generic function to pick up the particular implementation
> specified by an ordering annotation.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> Benno, please take a good and if you want to provide your Reviewed-by
> for this one. I didn't apply your Reviewed-by because I used
> `ordering::Any` instead of `AnyOrdering`, I think you're Ok with it [1],
> but I could be wrong. Thanks!
>
> [1]: https://lore.kernel.org/rust-for-linux/DB8M91D7KIT4.14W69YK7108ND@kernel.org/

> +/// The trait bound for annotating operations that support any ordering.
> +pub trait Any: internal::Sealed {

How about we just name this `Ordering`? Because that's what it is :)

That sadly means you can't do

    fn foo<Ordering: Ordering>() {}
           --------  ^^^^^^^^ not a trait
           |
           found this type parameter

But you can still do

    fn foo<O: Ordering>(_: O) {}

If we don't have the ordering module public and instead re-export from
atomic, you could also write:

    fn foo<Ordering: atomic::Ordering>(_: Ordering) {}

If you want it to be extra clear. What do you think?

---
Cheers,
Benno

> +    /// Describes the exact memory ordering.
> +    const TYPE: OrderingType;
> +}