Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

[Benno Lossin]

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 PM Wedson Almeida Filho <wedsonaf@xxxxxxxxx> wrote:
>>>>
>>>> From: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
>>>>
>>>> With GATs, we don't need a separate type to represent a borrowed object
>>>> 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@xxxxxxxxx>
>>>> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
>>>> Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
>>>> ---
>>>>    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 optimizations.
>>
> 
> 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 = 0;
     }
}

let arc = Arc::new(MutatingDrop { value: 42 });
let wr = arc.as_with_ref(); // this creates a shared `&` reference to the MutatingDrop
let arc2: Arc<MutatingDrop> = wr.into(); // increments the reference count 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.

-- 
Cheers,
Benno