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

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

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

> On Wed, Mar 05, 2025 at 06:24:56PM +0100, Andreas Hindborg wrote:
>> "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.
>
> Ultimately, if a struct implements AlwaysRefcounted, then you can always
> increments its refcount. If you want a version of the struct where that
> is not the case, then you need a different struct that does *not*
> implement AlwaysRefcounted.
>
> I do things like that in the mm_struct series. The VmaNew struct is an
> example of that.

Yea, I see your point. I think `AlwaysRefcounted` is just not going to
work for this use case. We can invent something new that suits our
needs.


Best regards,
Andreas Hindborg