[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <969eab7f-ad40-0dfb-18b9-6002fc54e12b@proton.me>
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
Powered by blists - more mailing lists