[RFC PATCH] rust: types: Add explanation for ARef pattern

[Boqun Feng <boqun.feng@gmail.com>]

As the usage of `ARef` and `AlwaysRefCounted` is growing, it makes sense
to add explanation of the "ARef pattern" to cover the most "DO" and "DO
NOT" cases when wrapping a self-refcounted C type.

Hence an "ARef pattern" section is added in the documentation of `ARef`.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
This is motivated by:

	https://lore.kernel.org/rust-for-linux/20240705110228.qqhhynbwwuwpcdeo@vireshk-i7/

 rust/kernel/types.rs | 156 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 156 insertions(+)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index bd189d646adb..70fdc780882e 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -329,6 +329,162 @@ pub unsafe trait AlwaysRefCounted {
 ///
 /// 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.
+///
+/// # [`ARef`] pattern
+///
+/// "[`ARef`] pattern" is preferred when wrapping a C struct which has its own refcounting
+/// mechanism, because it decouples the operations on the object itself (usually via a `&Foo`) vs the
+/// operations on a pointer to the object (usually via an `ARef<Foo>`). For example, given a `struct
+/// foo` defined in C, which has its own refcounting operations `get_foo()` and `put_foo()`. Without
+/// "[`ARef`] pattern", i.e. **bad case**:
+///
+/// ```ignore
+/// pub struct Foo(NonNull<foo>);
+///
+/// impl Foo {
+///     // An operation on the pointer.
+///     pub unsafe fn from_ptr(ptr: *mut foo) -> Self {
+///         // Note that whether `get_foo()` is needed here depends on the exact semantics of
+///         // `from_ptr()`: is it creating a new reference, or it continues using the caller's
+///         // reference?
+///         unsafe { get_foo(ptr); }
+///
+///         unsafe { Foo(NonNull::new_unchecked(foo)) }
+///     }
+///
+///     // An operation on the object.
+///     pub fn get_bar(&self) -> Bar {
+///         unsafe { (*foo.0.as_ptr()).bar }
+///     }
+/// }
+///
+/// // Plus `impl Clone` and `impl Drop` are also needed to implement manually.
+/// impl Clone for Foo {
+///     fn clone(&self) -> Self {
+///         unsafe { get_foo(self.0.as_ptr()); }
+///
+///         Foo(self.0)
+///     }
+/// }
+///
+/// impl Drop for Foo {
+///     fn drop(&mut self) {
+///         unsafe { put_foo(self.0.as_ptr()); }
+///     }
+/// }
+/// ```
+///
+/// In this case, it's hard to tell whether `Foo` represent an object of `foo` or a pointer to
+/// `foo`.
+///
+/// However, if using [`ARef`] pattern, `foo` can be wrapped as follow:
+///
+/// ```ignore
+/// /// Note: `Opaque` is needed in most cases since there usually exist C operations on
+/// /// `struct foo *`, and `#[repr(transparent)]` is needed for the safety of converting a `*mut
+/// /// foo` to a `*mut Foo`
+/// #[repr(transparent)]
+/// pub struct Foo(Opaque<foo>);
+///
+/// impl Foo {
+///     pub fn get_bar(&self) -> Bar {
+///         // SAFETY: `self.0.get()` is a valid pointer.
+///         //
+///         // Note: Usually extra safety comments are needed here to explain why accessing `.bar`
+///         // doesn't race with C side. Most cases are either calling a C function, which has its
+///         // own concurrent access protection, or holding a lock.
+///         unsafe { (*self.0.get()).bar }
+///     }
+/// }
+/// ```
+///
+/// ## Avoid `impl AlwaysRefCounted` if unnecesarry
+///
+/// If Rust code doesn't touch the part where the object lifetimes of `foo` are maintained, `impl
+/// AlwaysRefCounted` can be temporarily avoided: it can always be added later as an extension of
+/// the functionality of the Rust code. This is usually the case for callbacks where the object
+/// lifetimes are already maintained by a framework. In such a case, an `unsafe` `fn(*mut foo) ->
+/// &Foo` function usually suffices:
+///
+/// ```ignore
+/// impl Foo {
+///     /// # Safety
+///     ///
+///     /// `ptr` has to be a valid pointer to `foo` for the entire lifetime `'a'.
+///     pub unsafe fn as_ref<'a>(ptr: *mut foo) -> &'a Self {
+///         // SAFETY: Per function safety requirement, reborrow is valid.
+///         unsafe { &*ptr.cast() }
+///     }
+/// }
+/// ```
+///
+/// ## Type invariants of `impl AlwaysRefCounted`
+///
+/// Types that `impl AlwaysRefCounted` usually needs an invariant to describe why the type can meet
+/// the safety requirement of `AlwaysRefCounted`, e.g.
+///
+/// ```ignore
+/// /// # Invariants:
+/// ///
+/// /// Instances of this type are always refcounted, that is, a call to `get_foo` ensures that the
+/// /// allocation remains valid at least until the matching call to `put_foo`.
+/// #[repr(transparent)]
+/// pub struct Foo(Opaque<foo>);
+///
+/// // SAFETY: `Foo` is always ref-counted per type invariants.
+/// unsafe impl AlwaysRefCounted for Foo {
+///     fn inc_ref(&self) {
+///         // SAFETY: `self.0.get()` is a valid pointer and per type invariants, the existence of
+///         // `&self` means it has a non-zero reference count.
+///         unsafe { get_foo(self.0.get()); }
+///     }
+///
+///     unsafe dec_ref(obj: NonNull<Self>) {
+///         // SAFETY: The refcount of `obj` is non-zero per function safety requirement, and the
+///         // cast is OK since `foo` is transparent to `Foo`.
+///         unsafe { put_foo(obj.cast()); }
+///     }
+/// }
+/// ```
+///
+/// After `impl AlwaysRefCounted for foo`, `clone()` (`get_foo()`) and `drop()` (`put_foo()`)  are
+/// available to `ARef<Foo>` thanks to the generic implementation.
+///
+/// ## `ARef<Self>` vs `&Self`
+///
+/// For an `impl AlwaysRefCounted` type, `ARef<Self>` represents an owner of one reference count,
+/// e.g.
+///
+/// ```ignore
+/// impl Foo {
+///     /// Gets a ref-counted reference of [`Self`].
+///     ///
+///     /// # Safety
+///     ///
+///     /// - `ptr` must be a valid pointer to `foo` with at least one reference count.
+///     pub unsafe fn from_ptr(ptr: *mut foo) -> ARef<Self> {
+///         // SAFETY: `ptr` is a valid pointer per function safety requirement. The cast is OK
+///         // since `foo` is transparent to `Foo`.
+///         //
+///         // Note: `.into()` here increases the reference count, so the returned value has its own
+///         // reference count.
+///         unsafe { &*(ptr.cast::<Foo>()) }.into()
+///     }
+/// }
+/// ```
+///
+/// Another function that returns an `ARef<Self>` but with a different semantics is
+/// [`ARef::from_raw`]: it takes away the refcount of the input pointer, i.e. no refcount
+/// incrementation inside the function.
+///
+/// However `&Self` represents a reference to the object, and the lifetime of the **reference** is
+/// known at compile-time. E.g. the `Foo::as_ref()` above.
+///
+/// ## `impl Drop` of an `impl AlwaysRefCounted` should not touch the refcount
+///
+/// [`ARef`] descreases the refcount automatically (in [`ARef::drop`]) when it goes out of the
+/// scope, therefore there's no need to `impl Drop` for the type of objects (e.g. `Foo`) to decrease
+/// the refcount.
 pub struct ARef<T: AlwaysRefCounted> {
     ptr: NonNull<T>,
     _p: PhantomData<T>,
-- 
2.45.2