On Wed Aug 5, 2026 at 3:59 PM BST, Philipp Stanner wrote:

From: Danilo Krummrich dakr@kernel.org

Implement ForeignOwnable for ARef<T>, making it possible for C code to own an ARef<T>.

Since ARef represents shared ownership, BorrowedMut is &T rather than &mut T, matching the semantics of the underlying reference-counted type.

Signed-off-by: Danilo Krummrich dakr@kernel.org Reviewed-by: Alice Ryhl aliceryhl@google.com Tested-by: Daniel Almeida daniel.almeida@collabora.com

---

rust/kernel/sync/aref.rs | 40 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 40 insertions(+)

diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs index b721b2e00b98..540766613659 100644 --- a/rust/kernel/sync/aref.rs +++ b/rust/kernel/sync/aref.rs @@ -24,6 +24,11 @@ ptr::NonNull, // }; +use crate::{

• prelude::*,
• types::ForeignOwnable, //

+};

/// Types that are _always_ reference counted. /// /// It allows such types to define their own custom ref increment and decrement functions. @@ -188,6 +193,41 @@ fn eq(&self, other: &ARef<U>) -> bool { } impl<T: AlwaysRefCounted + Eq> Eq for ARef<T> {} +// SAFETY: `into_foreign` returns a pointer from `NonNull::as_ptr`, so it's non-null. The +// `ARef` invariant guarantees that `ptr` points to a valid `T`, so it's aligned to `T`. +unsafe impl<T: AlwaysRefCounted + 'static> ForeignOwnable for ARef<T> {

This doesn't need to be static, if you can add `where Self: 'a` on `Borrowed` and `BorrowedMut` instead.

Best, Gary

• const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
• type Borrowed<'a> = &'a T;
• type BorrowedMut<'a> = &'a T;
• fn into_foreign(self) -> *mut c_void {

•    ARef::into_raw(self).as_ptr().cast()
  

• }
• unsafe fn from_foreign(ptr: *mut c_void) -> Self {

•    // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
  

•    // call to `Self::into_foreign`.
  

•    let ptr = unsafe { NonNull::new_unchecked(ptr.cast()) };
  

•    // SAFETY: `ptr` came from `into_foreign`, which consumed an `ARef` without decrementing
  

•    // the refcount, so we can transfer the ownership to the new `ARef`.
  

•    unsafe { ARef::from_raw(ptr) }
  

• }
• unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T {

•    // SAFETY: The safety requirements of this method ensure that the object remains alive and
  

•    // immutable for the duration of 'a.
  

•    unsafe { &*ptr.cast() }
  

• }
• unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a T {

•    // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
  

•    // requirements for `borrow`.
  

•    unsafe { <Self as ForeignOwnable>::borrow(ptr) }
  

• }

+}

impl<T, U> PartialEq<&'_ U> for ARef<T> where T: AlwaysRefCounted + PartialEq<U>,