linux-kernel - Re: [PATCH] rust: sync: implement Unpin for ARef

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Message-Id: <4B26E76F-3863-47EC-8515-4B859ABFD82A@collabora.com>
Date: Wed, 17 Dec 2025 16:20:56 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: sync: implement Unpin for ARef



> On 17 Dec 2025, at 14:37, Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
> The default implementation of Unpin for ARef<T> is conditional on T
> being Unpin due to its PhantomData<T> field. However, this is overly
> strict as pointers to T are legal to move even if T itself cannot move.
> 
> Since commit 66f1ea83d9f8 ("rust: lock: Add a Pin<&mut T> accessor")
> this causes build failures when combined with a Mutex that contains an
> field ARef<T>, because almost any type that ARef is used with is !Unpin.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> rust/kernel/sync/aref.rs | 3 +++
> 1 file changed, 3 insertions(+)
> 
> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> index 0d24a0432015d09509a255df0a4f114171ae9fb0..c4be6f5d4884ddd6cbfbd7c27d396310a7cde30e 100644
> --- a/rust/kernel/sync/aref.rs
> +++ b/rust/kernel/sync/aref.rs
> @@ -83,6 +83,9 @@ unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
> // example, when the reference count reaches zero and `T` is dropped.
> unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
> 
> +// Even if T is pinned, pointers to T can still move.
> +impl<T> Unpin for ARef<T> {}
> +
> impl<T: AlwaysRefCounted> ARef<T> {
>     /// Creates a new instance of [`ARef`].
>     ///
> 
> ---
> base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
> change-id: 20251217-unpin-for-aref-3dc0fe3fd5b0
> 
> Best regards,
> -- 
> Alice Ryhl <aliceryhl@...gle.com>
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>