linux-kernel - Re: [PATCH v4] rust: move ARef and AlwaysRefCounted to sync::aref

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Message-Id: <DBCLH4WXYTJL.FDZ9B39OO3TY@kernel.org>
Date: Tue, 15 Jul 2025 13:33:18 +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 v4] rust: move ARef and AlwaysRefCounted to sync::aref

On Tue Jul 15, 2025 at 1:04 PM CEST, Shankari Anand wrote:
> Move the definitions of `ARef` and `AlwaysRefCounted` from `types.rs`
> to a new file `sync/aref.rs`.
> Define the corresponding `aref` module under `rust/kernel/sync.rs`.
> These types are better grouped in `sync`.
>
> To avoid breaking existing imports, they are re-exported from `types.rs`.
> Drop unused imports `mem::ManuallyDrop`, `ptr::NonNull` from `types.rs`,
> they are now only used in `sync/aref.rs`, where they are already imported.
>
> Suggested-by: Benno Lossin <lossin@...nel.org>
> Link: https://github.com/Rust-for-Linux/linux/issues/1173
> Signed-off-by: Shankari Anand <shankari.ak0208@...il.com>

One comment below, but that can be fixed when applying the patch.

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

> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> new file mode 100644
> index 000000000000..752eef31d45e
> --- /dev/null
> +++ b/rust/kernel/sync/aref.rs
> @@ -0,0 +1,154 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Built-in Reference Counting Support
> +
> +use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};
> +
> +/// 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.
> +

There is a missing `///` in this line, but Miguel can fix that when
picking the patch.

---
Cheers,
Benno

> +/// # 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 {