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Date:   Mon, 25 Sep 2023 15:07:44 +0000
To:     Boqun Feng <boqun.feng@gmail.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Alice Ryhl <aliceryhl@google.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        linux-kernel@vger.kernel.org,
        Wedson Almeida Filho <walmeida@microsoft.com>
Subject: Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`
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On 25.09.23 16:49, Boqun Feng wrote:
> On Mon, Sep 25, 2023 at 09:14:50AM +0000, Benno Lossin wrote:
>> On 25.09.23 08:29, Alice Ryhl wrote:
>>> On Sat, Sep 23, 2023 at 4:50=E2=80=AFPM Wedson Almeida Filho <wedsonaf@=
gmail.com> wrote:
>>>>
>>>> From: Wedson Almeida Filho <walmeida@microsoft.com>
>>>>
>>>> With GATs, we don't need a separate type to represent a borrowed objec=
t
>>>> with a refcount, we can just use Rust's regular shared borrowing. In
>>>> this case, we use `&WithRef<T>` instead of `ArcBorrow<'_, T>`.
>>>>
>>>> Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
>>>> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
>>>> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
>>>> ---
>>>>    rust/kernel/sync.rs     |   2 +-
>>>>    rust/kernel/sync/arc.rs | 134 ++++++++++++-------------------------=
---
>>>>    2 files changed, 39 insertions(+), 97 deletions(-)
>>>
>>> I'm concerned about this change, because an `&WithRef<T>` only has
>>> immutable permissions for the allocation. No pointer derived from it
>>> may be used to modify the value in the Arc, however, the drop
>>> implementation of Arc will do exactly that.
>>
>> That is indeed a problem. We could put the value in an `UnsafeCell`, but
>> that would lose us niche optimizations and probably also other optimizat=
ions.
>>
>=20
> Not sure I understand the problem here, why do we allow modifying the
> value in the Arc if you only have a shared ownership? Also I fail to see
> why `ArcBorrow` doesn't have the problem. Maybe I'm missing something
> subtle here? Could you provide an example?

Sure, here is the problem:

```rust
struct MutatingDrop {
     value: i32,
}

impl Drop for MutatingDrop {
     fn drop(&mut self) {
         self.value =3D 0;
     }
}

let arc =3D Arc::new(MutatingDrop { value: 42 });
let wr =3D arc.as_with_ref(); // this creates a shared `&` reference to the=
 MutatingDrop
let arc2: Arc<MutatingDrop> =3D wr.into(); // increments the reference coun=
t to 2
drop(arc); // this decrements the reference count to 1
drop(arc2); // this decrements the reference count to 0, so it will drop it
```
When dropping `arc2` it will run the destructor for `MutatingDrop`,
which mutates `value`. This is a problem, because the mutable reference
supplied was derived from a `&`, that is not allowed in Rust.

--=20
Cheers,
Benno