linux-kernel - Re: [PATCH v3 1/4] rust: move ARef and AlwaysRefCounted to sync::aref

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Message-Id: <DBBOQP2KELBM.IWOU5PB0KR27@kernel.org>
Date: Mon, 14 Jul 2025 11:54:10 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Shankari Anand" <shankari.ak0208@...il.com>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Cc: "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>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>
Subject: Re: [PATCH v3 1/4] rust: move ARef and AlwaysRefCounted to
 sync::aref

On Sun Jul 13, 2025 at 11:36 PM CEST, Shankari Anand wrote:
> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> new file mode 100644
> index 000000000000..c62dbb28282e
> --- /dev/null
> +++ b/rust/kernel/sync/aref.rs
> @@ -0,0 +1,192 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Built-in Reference Counting Support
> +
> +/// Types that are _always_ reference counted.
> +///
> +/// It allows such types to define their own custom ref increment and decrement functions.
> +/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
> +/// [`ARef<T>`].
> +///
> +/// This is usually implemented by wrappers to existing structures on the C side of the code. For
> +/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
> +/// instances of a type.
> +use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};

This line seems misplaced?

> +
> +/// # Safety
> +///
> +/// Implementers must ensure that increments to the reference count keep the object alive in memory
> +/// at least until matching decrements are performed.
> +///
> +/// Implementers must also ensure that all instances are reference-counted. (Otherwise they
> +/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
> +/// alive.)
> +pub unsafe trait AlwaysRefCounted {
> +    /// Increments the reference count on the object.
> +    fn inc_ref(&self);
> +
> +    /// Decrements the reference count on the object.
> +    ///
> +    /// Frees the object when the count reaches zero.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that there was a previous matching increment to the reference count,
> +    /// and that the object is no longer used after its reference count is decremented (as it may
> +    /// result in the object being freed), unless the caller owns another increment on the refcount
> +    /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
> +    /// [`AlwaysRefCounted::dec_ref`] once).
> +    unsafe fn dec_ref(obj: NonNull<Self>);
> +}

> +impl<T: AlwaysRefCounted> Drop for ARef<T> {
> +    fn drop(&mut self) {
> +        // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
> +        // decrement.
> +        unsafe { T::dec_ref(self.ptr) };
> +    }
> +}
> +
> +/// A sum type that always holds either a value of type `L` or `R`.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::types::Either;
> +///
> +/// let left_value: Either<i32, &str> = Either::Left(7);
> +/// let right_value: Either<i32, &str> = Either::Right("right value");
> +/// ```
> +pub enum Either<L, R> {
> +    /// Constructs an instance of [`Either`] containing a value of type `L`.
> +    Left(L),
> +
> +    /// Constructs an instance of [`Either`] containing a value of type `R`.
> +    Right(R),
> +}

What is the `Either` type doing here? Maybe a copy-paste mistake?

---
Cheers,
Benno