Hi Benno, Thanks for taking a look. On Thu, Jul 25, 2024 at 06:51:56PM +0000, Benno Lossin wrote: > 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"? > "its own" indicates the reference counters are inside the object (i.e. self refcounted), it's different than `Arc<T>` where the reference counters are "attached" to `T`. Your version looks good to me as well. > > +/// 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. > Basically to me, the advantages are mostly (1) and (2) in your list, thank you for the list. And I did try to use an example (below) to explain these, because I felt an example of the bad cases is straightforward. I will add your list here, because although an example may be straightforward of reading, a list of advantages are better for references. Again, thanks a lot! > > +/// 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:". > I'm OK with your version, but for my personal interest, why? ;-) > > +/// > > +/// ```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`). > hmm.. "since there usually exist C operations on ..." include these two cases you mentioned, no? Plus, the reference counters themselves are not marked as atomic at the moment, so without `Opaque`, we also have UB because of the reference counters. I was trying to summarize all these as "C operations on ...", maybe I should say "concurrent C operations on ..."? I am trying to be concise here since it's a comment inside a comment ;-) > > +/// /// `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. > These two sound good to me. Regards, Boqun > > +/// > > +/// 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 > > >
