On 10.07.24 05:24, Boqun Feng wrote: > 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@xxxxxxxxx> > --- > 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 I would have written "[...] struct which is reference-counted, because [...]", is there a specific reason you wrote "its own"? > +/// 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 Not exactly sure I understand your point here, what exactly is the advantage of decoupling the operations? In my mind the following points are the advantages of using `ARef`: (1) prevents having to implement multiple abstractions for a single C object: say there is a `struct foo` that is both used via reference counting and by-value on the stack. Without `ARef`, we would have to write two abstractions, one for each use-case. With `ARef`, we can have one `Foo` that can be wrapped with `ARef` to represent a reference-counted object. (2) `ARef<T>` always represents a reference counted object, so it helps with understanding the code. If you read `Foo`, you cannot be sure if it is heap or stack allocated. (3) generalizes common code of reference-counted objects (ie avoiding code duplication) and concentration of `unsafe` code. In my opinion (1) is the most important, then (2). And (3) is a nice bonus. If you agree with the list above (maybe you also have additional advantages of `ARef`?) then it would be great if you could also add them somewhere here. > +/// foo` defined in C, which has its own refcounting operations `get_foo()` and `put_foo()`. Without > +/// "[`ARef`] pattern", i.e. **bad case**: Instead of "bad case" I would have written "i.e. you want to avoid this:". > +/// > +/// ```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 I would disagree for the reason that `Opaque` is needed. You need it if the `foo` eg contains a bool, since C might just write a nonsense integer which would then result in immediate UB in Rust. Other reasons might be that certain bytes of `foo` are written to by other threads, even though on the Rust side we have `&mut Foo` (eg a `mutex`). > +/// /// `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 I would move this section below the next one. > +/// > +/// 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` I think you should first show how the example looks like with `ARef` and then talk about type invariants. > +/// > +/// 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 Typo: it should be `impl AlwaysRefCounted for Foo`. --- Cheers, Benno > +/// 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 >
