Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:3 as permitted sender) client-ip=2620:137:e000::3:3;
Date:   Mon, 25 Sep 2023 09:14:50 +0000
To:     Alice Ryhl <aliceryhl@google.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>
From:   Benno Lossin <benno.lossin@proton.me>
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>,
        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`
Message-ID: <969eab7f-ad40-0dfb-18b9-6002fc54e12b@proton.me>
In-Reply-To: <CAH5fLggxsewmtzXjehbawDCTHO0C7kteU_CLnh80eMNj=QyP9Q@mail.gmail.com>
References: <20230923144938.219517-1-wedsonaf@gmail.com> <20230923144938.219517-3-wedsonaf@gmail.com> <CAH5fLggxsewmtzXjehbawDCTHO0C7kteU_CLnh80eMNj=QyP9Q@mail.gmail.com>
Feedback-ID: 71780778:user:proton
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On 25.09.23 08:29, Alice Ryhl wrote:
> On Sat, Sep 23, 2023 at 4:50=E2=80=AFPM Wedson Almeida Filho <wedsonaf@gm=
ail.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(-)
>=20
> 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.=20

That is indeed a problem. We could put the value in an `UnsafeCell`, but
that would lose us niche optimizations and probably also other optimization=
s.

> 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>`) exi=
st.
So I think this would still be fine.

--=20
Cheers,
Benno