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

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Message-ID: <20251218-unpin-for-aref-v2-1-30d77129cbc6@google.com>
Date: Thu, 18 Dec 2025 08:25:13 +0000
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, Daniel Almeida <daniel.almeida@...labora.com>, 
	Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH v2] rust: sync: implement Unpin for ARef

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>
---
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(+)

diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index 0d24a0432015d09509a255df0a4f114171ae9fb0..a297477f23a3960928302709f6aeb322fd4d1bd0 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: AlwaysRefCounted> 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>