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

[Alice Ryhl]

On Fri, Jul 26, 2024 at 6:20 PM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
>
> On Fri, Jul 26, 2024 at 03:54:37PM +0000, Benno Lossin wrote:
> > On 26.07.24 17:15, Boqun Feng wrote:
> > > On Fri, Jul 26, 2024 at 02:42:36PM +0000, Benno Lossin wrote:
> > >> On 26.07.24 16:26, Boqun Feng wrote:
> > >>> On Fri, Jul 26, 2024 at 01:43:38PM +0000, Benno Lossin wrote:
> > >>> [...]
> > >>>>>>
> > >>>>>> You can always get a `&T` from `ARef<T>`, since it implements `Deref`.
> > >>>>>>
> > >>>>>
> > >>>>> Yeah, but this is unrelated. I was talking about that API providers can
> > >>>>> decide whether they want to only provide a `raw_ptr` -> `ARef<Self>` if
> > >>>>> they don't need to provide a `raw_ptr` -> `&Self`.
> > >>>>>
> > >>>>>>> Overall, I feel like we don't necessarily make a preference between
> > >>>>>>> `->&Self` and `->ARef<Self>` functions here, since it's up to the users'
> > >>>>>>> design?
> > >>>>>>
> > >>>>>> I would argue that there should be a clear preference for functions
> > >>>>>> returning `&Self` when possible (ie there is a parameter that the
> > >>>>>
> > >>>>> If "possible" also means there's going to be `raw_ptr` -> `&Self`
> > >>>>> function (as the same publicity level) anyway, then agreed. In other
> > >>>>> words, if the users only need the `raw_ptr` -> `ARef<Self>`
> > >>>>> functionality, we don't want to force people to provide a `raw_ptr` ->
> > >>>>> `&Self` just because, right?
> > >>>>
> > >>>> I see... I am having a hard time coming up with an example where users
> > >>>> would exclusively want `ARef<Self>` though... What do you have in mind?
> > >>>> Normally types wrapped by `ARef` have `&self` methods.
> > >>>>
> > >>>
> > >>> Having `&self` methods doesn't mean the necessarity of a `raw_ptr` ->
> > >>> `&Self` function, for example, a `Foo` is wrapped as follow:
> > >>>
> > >>>   struct Foo(Opaque<foo>);
> > >>>   impl Foo {
> > >>>       pub fn bar(&self) -> Bar { ... }
> > >>>       pub unsafe fn get_foo(ptr: *mut foo) -> ARef<Foo> { ... }
> > >>>   }
> > >>>
> > >>> in this case, the abstration provider may not want user to get a
> > >>> `raw_ptr` -> `&Self` function, so no need to have it.
> > >>
> > >> I don't understand this, why would the abstraction provider do that? The
> > >
> > > Because no user really needs to convert a `raw_ptr` to a `&Self` whose
> > > lifetime is limited to a scope?
> >
> > What if you have this:
> >
> >     unsafe extern "C" fn called_from_c_via_vtable(foo: *mut bindings::foo) {
> >         // SAFETY: ...
> >         let foo = unsafe { Foo::from_raw(foo) };
> >         foo.bar();
> >     }
> >
> > In this case, there is no need to take a refcount on `foo`.
> >
> > > Why do we provide a function if no one needs and the solely purpose is
> > > to just avoid providing another function?
> >
> > I don't think that there should be a lot of calls to that function
> > anyways and thus I don't think there is value in providing two functions
> > for almost the same behavior. Since one can be derived by the other, I
> > would go for only implementing the first one.
>
> I don't think there should be a rule saying that we can't provide a wrapper
> function for deriving an `ARef<T>`. `Device` is a good example:
>
> `let dev: ARef<Device> = unsafe { Device::from_raw(raw_dev) }.into();`
>
> vs.
>
> `let dev = unsafe { Device::get(raw_dev) };`
>
> To me personally, the latter looks quite a bit cleaner.
>
> Besides that, I think every kernel engineer (even without Rust background) will
> be able to decode the meaning of this call. And if we get the chance to make
> things obvious to everyone *without* the need to make a compromise, we should
> clearly take it.

I think I've come around on this question. I think it's fine to have
raw_ptr->ARef methods that increment the refcount, but we should make
a naming convention clear. I propose:

* Functions named things like from_raw_file or from_raw_mm do not
increment the refcount.
* Functions named things like get_file or or mmget do increment the
refcount, just like the C function of the same name.

Alice