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

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Date:   Mon, 25 Sep 2023 09:14:50 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

On 25.09.23 08:29, Alice Ryhl wrote:
> On Sat, Sep 23, 2023 at 4:50 PM Wedson Almeida Filho <wedsonaf@...il.com> wrote:
>>
>> From: Wedson Almeida Filho <walmeida@...rosoft.com>
>>
>> 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@...il.com>
>> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
>> Signed-off-by: Wedson Almeida Filho <walmeida@...rosoft.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 optimizations.

> It also means that we
> can't convert an Arc with refcount 1 into a UniqueArc.

I think you still can, since to do that you would consume the `Arc<T>` by
value, thus guaranteeing that no references (and thus no `&WithRef<T>`) exist.
So I think this would still be fine.

-- 
Cheers,
Benno