linux-kernel - Re: [PATCH v2] rust: sync: implement Unpin for ARef

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Message-Id: <DF19PO1J860C.1A15C38JSTK1J@kernel.org>
Date: Thu, 18 Dec 2025 11:24:47 +0100
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Miguel Ojeda" <ojeda@...nel.org>,
 "Boqun Feng" <boqun.feng@...il.com>
Cc: "Gary Guo" <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "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>, "Daniel
 Almeida" <daniel.almeida@...labora.com>
Subject: Re: [PATCH v2] rust: sync: implement Unpin for ARef

On Thu Dec 18, 2025 at 9:25 AM CET, Alice Ryhl 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.
>
> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

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

Cheers,
Benno

> ---
> Changes in v2:
> - Add T: AlwaysRefCounted bound.
> - Link to v1: https://lore.kernel.org/r/20251217-unpin-for-aref-v1-1-84711b747d02@google.com
> ---
>  rust/kernel/sync/aref.rs | 3 +++
>  1 file changed, 3 insertions(+)