[PATCH v18 7/8] rust: Add `OwnableRefCounted`

[Andreas Hindborg <a.hindborg@kernel.org>]

From: Oliver Mangold <oliver.mangold@pm.me>

Types implementing one of these traits can safely convert between an
`ARef<T>` and an `Owned<T>`.

This is useful for types which generally are accessed through an `ARef`
but have methods which can only safely be called when the reference is
unique, like e.g. `block::mq::Request::end_ok()`.

Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>
[ Andreas: Fix formatting, update documentation, fix error handling in
  examples. ]
Co-developed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/owned.rs     | 140 +++++++++++++++++++++++++++++++++++++++++++++--
 rust/kernel/sync/aref.rs |  16 +++++-
 rust/kernel/types.rs     |   1 +
 3 files changed, 151 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
index e79936c00002c..bb4223c0f725a 100644
--- a/rust/kernel/owned.rs
+++ b/rust/kernel/owned.rs
@@ -14,20 +14,26 @@
     pin::Pin,
     ptr::NonNull, //
 };
+use kernel::{
+    sync::aref::ARef,
+    types::RefCounted, //
+};
 
 use kernel::types::ForeignOwnable;
 
 /// Types that specify their own way of performing allocation and destruction. Typically, this trait
 /// is implemented on types from the C side.
 ///
-/// Implementing this trait allows types to be referenced via the [`Owned<Self>`] pointer type. This
-/// is useful when it is desirable to tie the lifetime of the reference to an owned object, rather
-/// than pass around a bare reference. [`Ownable`] types can define custom drop logic that is
-/// executed when the owned reference [`Owned<Self>`] pointing to the object is dropped.
+/// Implementing this trait allows types to be referenced via the [`Owned<Self>`] pointer type.
+///  - This is useful when it is desirable to tie the lifetime of an object reference to an owned
+///    object, rather than pass around a bare reference.
+///  - [`Ownable`] types can define custom drop logic that is executed when the owned reference
+///    of type [`Owned<_>`] pointing to the object is dropped.
 ///
 /// Note: The underlying object is not required to provide internal reference counting, because it
 /// represents a unique, owned reference. If reference counting (on the Rust side) is required,
-/// [`RefCounted`](crate::types::RefCounted) should be implemented.
+/// [`RefCounted`] should be implemented. [`OwnableRefCounted`] should be implemented if conversion
+/// between unique and shared (reference counted) ownership is needed.
 ///
 /// # Examples
 ///
@@ -239,3 +245,127 @@ unsafe fn borrow_mut<'a>(ptr: *mut kernel::ffi::c_void) -> Self::BorrowedMut<'a>
         unsafe { Pin::new_unchecked(inner) }
     }
 }
+
+/// A trait for objects that can be wrapped in either one of the reference types [`Owned`] and
+/// [`ARef`].
+///
+/// # Examples
+///
+/// A minimal example implementation of [`OwnableRefCounted`], [`Ownable`] and its usage with
+/// [`ARef`] and [`Owned`] looks like this:
+///
+/// ```
+/// # #![expect(clippy::disallowed_names)]
+/// # use core::cell::Cell;
+/// # use core::ptr::NonNull;
+/// # use kernel::alloc::{flags, kbox::KBox, AllocError};
+/// # use kernel::sync::aref::{ARef, RefCounted};
+/// # use kernel::types::{Owned, Ownable, OwnableRefCounted};
+///
+/// // An internally refcounted struct for demonstration purposes.
+/// //
+/// // # Invariants
+/// //
+/// // - `refcount` is always non-zero for a valid object.
+/// // - `refcount` is >1 if there is more than one Rust reference to it.
+/// //
+/// struct Foo {
+///     refcount: Cell<usize>,
+/// }
+///
+/// impl Foo {
+///     fn new() -> Result<Owned<Self>> {
+///         // We are just using a `KBox` here to handle the actual allocation, as our `Foo` is
+///         // not actually a C-allocated object.
+///         // INVARIANT: We initialize `refcount` to 1, satisfying the invariants.
+///         let result = KBox::new(
+///             Foo {
+///                 refcount: Cell::new(1),
+///             },
+///             flags::GFP_KERNEL,
+///         )?;
+///         let result = KBox::into_non_null(result);
+///         // SAFETY:
+///         //  - We just allocated the `Self`, thus it is valid and we own it.
+///         //  - We can transfer this ownership to the `from_raw` method.
+///         Ok(unsafe { Owned::from_raw(result) })
+///     }
+/// }
+///
+/// // SAFETY: We increment and decrement each time the respective function is called and only free
+/// // the `Foo` when the refcount reaches zero.
+/// unsafe impl RefCounted for Foo {
+///     fn inc_ref(&self) {
+///         self.refcount.replace(self.refcount.get() + 1);
+///     }
+///
+///     unsafe fn dec_ref(this: NonNull<Self>) {
+///         // SAFETY: By requirement on calling this function, the refcount is non-zero,
+///         // implying the underlying object is valid.
+///         let refcount = unsafe { &this.as_ref().refcount };
+///         let new_refcount = refcount.get() - 1;
+///         if new_refcount == 0 {
+///             // The `Foo` will be dropped when `KBox` goes out of scope.
+///             // SAFETY: The [`KBox<Foo>`] is still alive as the old refcount is 1. We can pass
+///             // ownership to the [`KBox`] as by requirement on calling this function,
+///             // the `Self` will no longer be used by the caller.
+///             unsafe { KBox::from_raw(this.as_ptr()) };
+///         } else {
+///             refcount.replace(new_refcount);
+///         }
+///     }
+/// }
+///
+/// impl OwnableRefCounted for Foo {
+///     fn try_from_shared(this: ARef<Self>) -> Result<Owned<Self>, ARef<Self>> {
+///         if this.refcount.get() == 1 {
+///             // SAFETY: The `Foo` is still alive and has no other Rust references as the refcount
+///             // is 1.
+///             Ok(unsafe { Owned::from_raw(ARef::into_raw(this)) })
+///         } else {
+///             Err(this)
+///         }
+///     }
+/// }
+///
+/// impl Ownable for Foo {
+///     unsafe fn release(this: NonNull<Self>) {
+///         // SAFETY: Using `dec_ref()` from [`RefCounted`] to release is okay, as the refcount is
+///         // always 1 for an [`Owned<Foo>`].
+///         unsafe { Foo::dec_ref(this) };
+///     }
+/// }
+///
+/// let foo = Foo::new()?;
+/// let foo = ARef::from(foo);
+/// {
+///     let bar = foo.clone();
+///     assert!(Owned::try_from(bar).is_err());
+/// }
+/// assert!(Owned::try_from(foo).is_ok());
+/// # Ok::<(), Error>(())
+/// ```
+pub trait OwnableRefCounted: RefCounted + Ownable + Sized {
+    /// Checks if the [`ARef`] is unique and converts it to an [`Owned`] if that is the case.
+    /// Otherwise it returns again an [`ARef`] to the same underlying object.
+    fn try_from_shared(this: ARef<Self>) -> Result<Owned<Self>, ARef<Self>>;
+
+    /// Converts the [`Owned`] into an [`ARef`].
+    #[inline]
+    fn into_shared(this: Owned<Self>) -> ARef<Self> {
+        // SAFETY: `Owned::into_raw` returns a pointer to a valid `Self`, and the `Owned` owned the
+        // reference count that we now transfer to the new `ARef`.
+        unsafe { ARef::from_raw(Owned::into_raw(this)) }
+    }
+}
+
+impl<T: OwnableRefCounted> TryFrom<ARef<T>> for Owned<T> {
+    type Error = ARef<T>;
+    /// Tries to convert the [`ARef`] to an [`Owned`] by calling
+    /// [`try_from_shared()`](OwnableRefCounted::try_from_shared). In case the [`ARef`] is not
+    /// unique, it returns again an [`ARef`] to the same underlying object.
+    #[inline]
+    fn try_from(b: ARef<T>) -> Result<Owned<T>, Self::Error> {
+        T::try_from_shared(b)
+    }
+}
diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index d0865aeb9371b..77eb390139079 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -23,6 +23,10 @@
     ops::Deref,
     ptr::NonNull, //
 };
+use kernel::types::{
+    OwnableRefCounted,
+    Owned, //
+};
 
 /// Types that are internally reference counted.
 ///
@@ -35,7 +39,10 @@
 /// Note: Implementing this trait allows types to be wrapped in an [`ARef<Self>`]. It requires an
 /// internal reference count and provides only shared references. If unique references are required
 /// [`Ownable`](crate::types::Ownable) should be implemented which allows types to be wrapped in an
-/// [`Owned<Self>`](crate::types::Owned).
+/// [`Owned<Self>`](crate::types::Owned). Implementing the trait
+/// [`OwnableRefCounted`] allows to convert between unique and
+/// shared references (i.e. [`Owned<Self>`](crate::types::Owned) and
+/// [`ARef<Self>`](crate::types::Owned)).
 ///
 /// # Safety
 ///
@@ -188,6 +195,13 @@ fn from(b: &T) -> Self {
     }
 }
 
+impl<T: OwnableRefCounted> From<Owned<T>> for ARef<T> {
+    #[inline]
+    fn from(b: Owned<T>) -> Self {
+        T::into_shared(b)
+    }
+}
+
 impl<T: RefCounted> Drop for ARef<T> {
     fn drop(&mut self) {
         // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 5ef763717e59a..6aa760952cb63 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -18,6 +18,7 @@
 pub use crate::{
     owned::{
         Ownable,
+        OwnableRefCounted,
         Owned, //
     },
     sync::aref::{

-- 
2.51.2