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

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Message-Id: <DB6JVOZLCMBL.3EZQJP50UUB86@kernel.org>
Date: Tue, 08 Jul 2025 11:02:17 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Andreas Hindborg" <a.hindborg@...nel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Masahiro Yamada" <masahiroy@...nel.org>, "Nathan
 Chancellor" <nathan@...nel.org>, "Luis Chamberlain" <mcgrof@...nel.org>,
 "Danilo Krummrich" <dakr@...nel.org>, "Nicolas Schier"
 <nicolas.schier@...ux.dev>
Cc: "Trevor Gross" <tmgross@...ch.edu>, "Adam Bratschi-Kaye"
 <ark.email@...il.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-kbuild@...r.kernel.org>, "Petr
 Pavlu" <petr.pavlu@...e.com>, "Sami Tolvanen" <samitolvanen@...gle.com>,
 "Daniel Gomez" <da.gomez@...sung.com>, "Simona Vetter"
 <simona.vetter@...ll.ch>, "Greg KH" <gregkh@...uxfoundation.org>, "Fiona
 Behrens" <me@...enk.dev>, "Daniel Almeida" <daniel.almeida@...labora.com>,
 <linux-modules@...r.kernel.org>
Subject: Re: [PATCH v15 1/7] rust: sync: add `SetOnce`

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@...nel.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@...nel.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
> +        }
> +    }