linux-kernel - Re: [PATCH] rust: sync: extend module documentation of aref

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Message-ID: <9bae0d3c-7900-474a-a0af-d2f90ec65012@sedlak.dev>
Date: Wed, 23 Jul 2025 12:25:49 +0200
From: Daniel Sedlak <daniel@...lak.dev>
To: Benno Lossin <lossin@...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>, Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 Shankari Anand <shankari.ak0208@...il.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: sync: extend module documentation of aref

Hi Benno,

On 7/22/25 2:14 PM, Benno Lossin wrote:
> Commit 07dad44aa9a9 ("rust: kernel: move ARef and AlwaysRefCounted to
> sync::aref") moved `ARef` and `AlwaysRefCounted` into their own module.
> In that process only a short, single line description of the module was
> added. Extend the description by explaining what is meant by "internal
> reference counting", the two items in the trait & the difference to
> `Arc`.
> 
> Signed-off-by: Benno Lossin <lossin@...nel.org>
> ---
>   rust/kernel/sync/aref.rs | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> index dbd77bb68617..1c212238c0e5 100644
> --- a/rust/kernel/sync/aref.rs
> +++ b/rust/kernel/sync/aref.rs
> @@ -1,6 +1,21 @@
>   // SPDX-License-Identifier: GPL-2.0
>   
>   //! Internal reference counting support.
> +//!
> +//! Many C types already have their own reference counting mechanism (e.g. by storing a
> +//! `refcount_t`). This module provides support for directly using their internal reference count
> +//! from Rust; instead of making users have to use an additional Rust-reference count in the form of
> +//! [`Arc`].
> +//!
> +//! The smart pointer [`ARef<T>`] acts similarly to [`Arc<T>`] in that it holds a refcount on the
> +//! underlying object, but this refcount is internal to the object. It essentially is a Rust
> +//! implementation of the `get_` and `put_` pattern used in C for reference counting.
> +//!
> +//! To make use of [`ARef<MyType>`], `MyType` needs to implement [`AlwaysRefCounted`]. It is a trait
> +//! for accessing the internal reference count of an object of the `MyType` type.
> +//!
> +//! [`Arc`]: crate::sync::Arc
> +//! [`Arc<T>`]: crate::sync::Arc

It got me curios. Why is it required to declare the doc reference for 
Arc and Arc<T>, but not ARef<MyType> and ARef<T>?

Is it because ARef is in file scope but not the Arc?

If so, you could just add

	use crate::sync::Arc;

in the file imports and you wouldn't have to duplicate the

	//! [`Arc`]: crate::sync::Arc
	//! [`Arc<T>`]: crate::sync::Arc

And even cleanup it a bit by simplifying

	[`Arc`](crate::sync::Arc) to [`Arc`]

in the AlwaysRefCounted trait.

Thanks!
Daniel