rust: kernel: move ARef and AlwaysRefCounted to sync::aref

Move the definitions of `ARef` and `AlwaysRefCounted` from `types.rs`
to a new file `sync/aref.rs`.  Define the corresponding `aref` module
under `rust/kernel/sync.rs`.  These types are better grouped in `sync`.

To avoid breaking existing imports, they are re-exported from `types.rs`.
Drop unused imports `mem::ManuallyDrop`, `ptr::NonNull` from `types.rs`,
they are now only used in `sync/aref.rs`, where they are already imported.

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1173
Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250715110423.334744-1-shankari.ak0208@gmail.com
[ Added missing `///`. Changed module title. Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

3 files changed, 158 insertions, 151 deletions

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 096cb78c8ec3..00f9b558a3ad 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -10,6 +10,7 @@ use crate::types::Opaque;
 use pin_init;
 
 mod arc;
+pub mod aref;
 pub mod completion;
 mod condvar;
 pub mod lock;
diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
new file mode 100644
index 000000000000..dbd77bb68617
--- /dev/null
+++ b/rust/kernel/sync/aref.rs
@@ -0,0 +1,154 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Internal reference counting support.
+
+use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};
+
+/// Types that are _always_ reference counted.
+///
+/// It allows such types to define their own custom ref increment and decrement functions.
+/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
+/// [`ARef<T>`].
+///
+/// This is usually implemented by wrappers to existing structures on the C side of the code. For
+/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
+/// instances of a type.
+///
+/// # Safety
+///
+/// Implementers must ensure that increments to the reference count keep the object alive in memory
+/// at least until matching decrements are performed.
+///
+/// Implementers must also ensure that all instances are reference-counted. (Otherwise they
+/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
+/// alive.)
+pub unsafe trait AlwaysRefCounted {
+    /// Increments the reference count on the object.
+    fn inc_ref(&self);
+
+    /// Decrements the reference count on the object.
+    ///
+    /// Frees the object when the count reaches zero.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that there was a previous matching increment to the reference count,
+    /// and that the object is no longer used after its reference count is decremented (as it may
+    /// result in the object being freed), unless the caller owns another increment on the refcount
+    /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
+    /// [`AlwaysRefCounted::dec_ref`] once).
+    unsafe fn dec_ref(obj: NonNull<Self>);
+}
+
+/// An owned reference to an always-reference-counted object.
+///
+/// The object's reference count is automatically decremented when an instance of [`ARef`] is
+/// dropped. It is also automatically incremented when a new instance is created via
+/// [`ARef::clone`].
+///
+/// # Invariants
+///
+/// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
+/// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
+pub struct ARef<T: AlwaysRefCounted> {
+    ptr: NonNull<T>,
+    _p: PhantomData<T>,
+}
+
+// SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because
+// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
+// `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
+// mutable reference, for example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
+
+// SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
+// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
+// it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
+// `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
+// example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
+
+impl<T: AlwaysRefCounted> ARef<T> {
+    /// Creates a new instance of [`ARef`].
+    ///
+    /// It takes over an increment of the reference count on the underlying object.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that the reference count was incremented at least once, and that they
+    /// are properly relinquishing one increment. That is, if there is only one increment, callers
+    /// must not use the underlying object anymore -- it is only safe to do so via the newly
+    /// created [`ARef`].
+    pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
+        // INVARIANT: The safety requirements guarantee that the new instance now owns the
+        // increment on the refcount.
+        Self {
+            ptr,
+            _p: PhantomData,
+        }
+    }
+
+    /// Consumes the `ARef`, returning a raw pointer.
+    ///
+    /// This function does not change the refcount. After calling this function, the caller is
+    /// responsible for the refcount previously managed by the `ARef`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use core::ptr::NonNull;
+    /// use kernel::types::{ARef, AlwaysRefCounted};
+    ///
+    /// struct Empty {}
+    ///
+    /// # // SAFETY: TODO.
+    /// unsafe impl AlwaysRefCounted for Empty {
+    ///     fn inc_ref(&self) {}
+    ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
+    /// }
+    ///
+    /// let mut data = Empty {};
+    /// let ptr = NonNull::<Empty>::new(&mut data).unwrap();
+    /// # // SAFETY: TODO.
+    /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
+    /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
+    ///
+    /// assert_eq!(ptr, raw_ptr);
+    /// ```
+    pub fn into_raw(me: Self) -> NonNull<T> {
+        ManuallyDrop::new(me).ptr
+    }
+}
+
+impl<T: AlwaysRefCounted> Clone for ARef<T> {
+    fn clone(&self) -> Self {
+        self.inc_ref();
+        // SAFETY: We just incremented the refcount above.
+        unsafe { Self::from_raw(self.ptr) }
+    }
+}
+
+impl<T: AlwaysRefCounted> Deref for ARef<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The type invariants guarantee that the object is valid.
+        unsafe { self.ptr.as_ref() }
+    }
+}
+
+impl<T: AlwaysRefCounted> From<&T> for ARef<T> {
+    fn from(b: &T) -> Self {
+        b.inc_ref();
+        // SAFETY: We just incremented the refcount above.
+        unsafe { Self::from_raw(NonNull::from(b)) }
+    }
+}
+
+impl<T: AlwaysRefCounted> Drop for ARef<T> {
+    fn drop(&mut self) {
+        // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
+        // decrement.
+        unsafe { T::dec_ref(self.ptr) };
+    }
+}
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 82b9cfeb4739..ec82a163cb0e 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -6,12 +6,13 @@ use crate::ffi::c_void;
 use core::{
     cell::UnsafeCell,
     marker::{PhantomData, PhantomPinned},
-    mem::{ManuallyDrop, MaybeUninit},
+    mem::MaybeUninit,
     ops::{Deref, DerefMut},
-    ptr::NonNull,
 };
 use pin_init::{PinInit, Zeroable};
 
+pub use crate::sync::aref::{ARef, AlwaysRefCounted};
+
 /// Used to transfer ownership to and from foreign (non-Rust) languages.
 ///
 /// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
@@ -420,155 +421,6 @@ impl<T> Opaque<T> {
     }
 }
 
-/// Types that are _always_ reference counted.
-///
-/// It allows such types to define their own custom ref increment and decrement functions.
-/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
-/// [`ARef<T>`].
-///
-/// This is usually implemented by wrappers to existing structures on the C side of the code. For
-/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
-/// instances of a type.
-///
-/// # Safety
-///
-/// Implementers must ensure that increments to the reference count keep the object alive in memory
-/// at least until matching decrements are performed.
-///
-/// Implementers must also ensure that all instances are reference-counted. (Otherwise they
-/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
-/// alive.)
-pub unsafe trait AlwaysRefCounted {
-    /// Increments the reference count on the object.
-    fn inc_ref(&self);
-
-    /// Decrements the reference count on the object.
-    ///
-    /// Frees the object when the count reaches zero.
-    ///
-    /// # Safety
-    ///
-    /// Callers must ensure that there was a previous matching increment to the reference count,
-    /// and that the object is no longer used after its reference count is decremented (as it may
-    /// result in the object being freed), unless the caller owns another increment on the refcount
-    /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
-    /// [`AlwaysRefCounted::dec_ref`] once).
-    unsafe fn dec_ref(obj: NonNull<Self>);
-}
-
-/// An owned reference to an always-reference-counted object.
-///
-/// The object's reference count is automatically decremented when an instance of [`ARef`] is
-/// dropped. It is also automatically incremented when a new instance is created via
-/// [`ARef::clone`].
-///
-/// # Invariants
-///
-/// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
-/// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
-pub struct ARef<T: AlwaysRefCounted> {
-    ptr: NonNull<T>,
-    _p: PhantomData<T>,
-}
-
-// SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because
-// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
-// `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
-// mutable reference, for example, when the reference count reaches zero and `T` is dropped.
-unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
-
-// SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
-// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
-// it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
-// `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
-// example, when the reference count reaches zero and `T` is dropped.
-unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
-
-impl<T: AlwaysRefCounted> ARef<T> {
-    /// Creates a new instance of [`ARef`].
-    ///
-    /// It takes over an increment of the reference count on the underlying object.
-    ///
-    /// # Safety
-    ///
-    /// Callers must ensure that the reference count was incremented at least once, and that they
-    /// are properly relinquishing one increment. That is, if there is only one increment, callers
-    /// must not use the underlying object anymore -- it is only safe to do so via the newly
-    /// created [`ARef`].
-    pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
-        // INVARIANT: The safety requirements guarantee that the new instance now owns the
-        // increment on the refcount.
-        Self {
-            ptr,
-            _p: PhantomData,
-        }
-    }
-
-    /// Consumes the `ARef`, returning a raw pointer.
-    ///
-    /// This function does not change the refcount. After calling this function, the caller is
-    /// responsible for the refcount previously managed by the `ARef`.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use core::ptr::NonNull;
-    /// use kernel::types::{ARef, AlwaysRefCounted};
-    ///
-    /// struct Empty {}
-    ///
-    /// # // SAFETY: TODO.
-    /// unsafe impl AlwaysRefCounted for Empty {
-    ///     fn inc_ref(&self) {}
-    ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
-    /// }
-    ///
-    /// let mut data = Empty {};
-    /// let ptr = NonNull::<Empty>::new(&mut data).unwrap();
-    /// # // SAFETY: TODO.
-    /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
-    /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
-    ///
-    /// assert_eq!(ptr, raw_ptr);
-    /// ```
-    pub fn into_raw(me: Self) -> NonNull<T> {
-        ManuallyDrop::new(me).ptr
-    }
-}
-
-impl<T: AlwaysRefCounted> Clone for ARef<T> {
-    fn clone(&self) -> Self {
-        self.inc_ref();
-        // SAFETY: We just incremented the refcount above.
-        unsafe { Self::from_raw(self.ptr) }
-    }
-}
-
-impl<T: AlwaysRefCounted> Deref for ARef<T> {
-    type Target = T;
-
-    fn deref(&self) -> &Self::Target {
-        // SAFETY: The type invariants guarantee that the object is valid.
-        unsafe { self.ptr.as_ref() }
-    }
-}
-
-impl<T: AlwaysRefCounted> From<&T> for ARef<T> {
-    fn from(b: &T) -> Self {
-        b.inc_ref();
-        // SAFETY: We just incremented the refcount above.
-        unsafe { Self::from_raw(NonNull::from(b)) }
-    }
-}
-
-impl<T: AlwaysRefCounted> Drop for ARef<T> {
-    fn drop(&mut self) {
-        // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
-        // decrement.
-        unsafe { T::dec_ref(self.ptr) };
-    }
-}
-
 /// Zero-sized type to mark types not [`Send`].
 ///
 /// Add this type as a field to your struct if your type should not be sent to a different task.