rust: sync: extend module documentation of aref

Commit 07dad44aa9a9 ("rust: kernel: move ARef and AlwaysRefCounted to
sync::aref") moved `ARef` and `AlwaysRefCounted` into their own module.
In that process only a short, single line description of the module was
added. Extend the description by explaining what is meant by "internal
reference counting", the two items in the trait & the difference to
`Arc`.

Signed-off-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index c53d42cb0734..0d24a0432015 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -1,6 +1,21 @@
 // SPDX-License-Identifier: GPL-2.0
 
 //! Internal reference counting support.
+//!
+//! Many C types already have their own reference counting mechanism (e.g. by storing a
+//! `refcount_t`). This module provides support for directly using their internal reference count
+//! from Rust; instead of making users have to use an additional Rust-reference count in the form of
+//! [`Arc`].
+//!
+//! The smart pointer [`ARef<T>`] acts similarly to [`Arc<T>`] in that it holds a refcount on the
+//! underlying object, but this refcount is internal to the object. It essentially is a Rust
+//! implementation of the `get_` and `put_` pattern used in C for reference counting.
+//!
+//! To make use of [`ARef<MyType>`], `MyType` needs to implement [`AlwaysRefCounted`]. It is a trait
+//! for accessing the internal reference count of an object of the `MyType` type.
+//!
+//! [`Arc`]: crate::sync::Arc
+//! [`Arc<T>`]: crate::sync::Arc
 
 use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};