Re: [PATCH v15 1/7] rust: sync: add `SetOnce`

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

On Mon Jul 7, 2025 at 3:29 PM CEST, Andreas Hindborg wrote:
> Introduce the `SetOnce` type, a container that can only be written once.
> The container uses an internal atomic to synchronize writes to the internal
> value.
>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

One nit and a safety comment fix below. (feel free to ignore the nit)
With the safety comment fixed:

Reviewed-by: Benno Lossin <lossin@kernel.org>

> ---
>  rust/kernel/sync.rs          |   2 +
>  rust/kernel/sync/set_once.rs | 125 +++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 127 insertions(+)
>
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 81e3a806e57e2..13e6bc7fa87ac 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -18,6 +18,7 @@
>  mod locked_by;
>  pub mod poll;
>  pub mod rcu;
> +mod set_once;

I would have named this `once`.

>  pub use arc::{Arc, ArcBorrow, UniqueArc};
>  pub use completion::Completion;

> +    /// Get a reference to the contained object.
> +    ///
> +    /// Returns [`None`] if this [`SetOnce`] is empty.
> +    pub fn as_ref(&self) -> Option<&T> {
> +        if self.init.load(Acquire) == 2 {
> +            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
> +            // contains a valid value.

And the type invariants also ensure that the value of `self.init`
doesn't change.

So probably

    // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
    // contains a valid value. They also guarantee that `self.init` doesn't change.

If you come up with something better, feel free to use it.

---
Cheers,
Benno

> +            Some(unsafe { &*self.value.get() })
> +        } else {
> +            None
> +        }
> +    }