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From:   Alice Ryhl <aliceryhl@google.com>
Date:   Mon, 25 Sep 2023 08:29:13 +0200
Message-ID: <CAH5fLggxsewmtzXjehbawDCTHO0C7kteU_CLnh80eMNj=QyP9Q@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`
To:     Wedson Almeida Filho <wedsonaf@gmail.com>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        linux-kernel@vger.kernel.org,
        Wedson Almeida Filho <walmeida@microsoft.com>
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On Sat, Sep 23, 2023 at 4:50=E2=80=AFPM Wedson Almeida Filho <wedsonaf@gmai=
l.com> wrote:
>
> From: Wedson Almeida Filho <walmeida@microsoft.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@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. It also means that we
can't convert an Arc with refcount 1 into a UniqueArc.

Alice