Re: [PATCH v4] rust: adding UniqueRefCounted and UniqueRef types

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alice Ryhl" <aliceryhl@google.com> writes:

> On Wed, Mar 5, 2025 at 4:39 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> "Alice Ryhl" <aliceryhl@google.com> writes:
>>
>> > On Wed, Mar 5, 2025 at 3:56 PM Oliver Mangold <oliver.mangold@pm.me> wrote:
>> >>
>> >> Hi Alice,
>> >>
>> >> On 250305 1339, Alice Ryhl wrote:
>> >> > On Wed, Mar 05, 2025 at 11:31:44AM +0000, Oliver Mangold wrote:
>> >> >
>> >> > > +impl<T: UniqueRefCounted> Deref for UniqueRef<T> {
>> >> > > +    type Target = T;
>> >> > > +
>> >> > > +    fn deref(&self) -> &Self::Target {
>> >> > > +        // SAFETY: The type invariants guarantee that the object is valid.
>> >> > > +        unsafe { self.ptr.as_ref() }
>> >> > > +    }
>> >> > > +}
>> >> >
>> >> > What stops people from doing this?
>> >> >
>> >> > let my_unique: UniqueRef<T> = ...;
>> >> > let my_ref: &T = &*my_unique;
>> >> > let my_shared: ARef<T> = ARef::from(my_ref);
>> >> >
>> >> > Now it is no longer unique.
>> >> >
>> >> Oh, indeed. That's a serious problem. I see 2 options to deal with that:
>> >>
>> >> 1. remove ARef::From<&T>
>> >>
>> >> I checked the users of this, and it looks to me like there is rather
>> >> a limited number and they are easy to fix by replacing the &T with ARef<T>.
>> >> But I assume that wouldn't be welcome as it is intrusive nonetheless
>> >> and of course there is ergonomic value in having the function around.
>> >
>> > Definitely not an option. There are many users of this function that
>> > are in the process of being upstreamed. The ability to go &T ->
>> > ARef<T> is pretty fundamental for ARef.
>>
>> Not having `impl From<&T> for UniqueArc` seems to work out fine.
>>
>> It would be unfortunate if `impl From<&T> for ARef<T>` would prevent us
>> from having a unique version of `ARef`. I would say that is a valid
>> reason to consider removing that impl.
>
> I think the impl is really important. It's required to do things such as:
>
> let mm = ARef::from(&*current!().mm());
>
> Without the impl (or something equivalent), it's not possible to
> increment the refcount of the &Mm returned by `current!().mm()`. There
> are many other examples of this.

Right. Let's see what we can figure out of other solutions then.


Best regards,
Andreas Hindborg