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

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Date:   Mon, 25 Sep 2023 08:29:13 +0200
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     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>,
        Benno Lossin <benno.lossin@...ton.me>,
        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 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. It also means that we
can't convert an Arc with refcount 1 into a UniqueArc.

Alice