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

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Message-ID: <CAH5fLggn_xwfoyct5b26PkGgWAhad6y1k5NUCsATP4f_0+_RzA@mail.gmail.com>
Date: Thu, 18 Dec 2025 09:34:24 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Cc: 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 Wed, Dec 17, 2025 at 6:37 PM 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> {}

This needs an AlwaysRefCounted bound. I sent a v2:

https://lore.kernel.org/all/20251218-unpin-for-aref-v2-1-30d77129cbc6@google.com/

Alice