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Date: Thu, 04 Apr 2024 13:28:16 +0000
To: Alice Ryhl <aliceryhl@google.com>
From: Benno Lossin <benno.lossin@proton.me>
Cc: Miguel Ojeda <ojeda@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Alex Gaynor <alex.gaynor@gmail.com>, Wedson Almeida Filho <wedsonaf@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>, Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>, Coly Li <colyli@suse.de>, Paolo Abeni <pabeni@redhat.com>, Pierre Gondois <pierre.gondois@arm.com>, Ingo Molnar <mingo@kernel.org>, Jakub Kicinski <kuba@kernel.org>, Wei Yang <richard.weiyang@gmail.com>, Matthew Wilcox <willy@infradead.org>, linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org
Subject: Re: [PATCH 7/9] rust: list: add cursor
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On 03.04.24 14:49, Alice Ryhl wrote:
> On Wed, Apr 3, 2024 at 2:19=E2=80=AFPM Benno Lossin <benno.lossin@proton.=
me> wrote:
>> On 02.04.24 14:17, Alice Ryhl wrote:
>>> +impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Cursor<'a, T, ID> {
>>> +    /// Access the current element of this cursor.
>>> +    pub fn current(&self) -> ArcBorrow<'_, T> {
>>> +        // SAFETY: The `current` pointer points a value in the list.
>>> +        let me =3D unsafe { T::view_value(ListLinks::from_fields(self.=
current)) };
>>> +        // SAFETY:
>>> +        // * All values in a list are stored in an `Arc`.
>>> +        // * The value cannot be removed from the list for the duratio=
n of the lifetime annotated
>>> +        //   on the returned `ArcBorrow`, because removing it from the=
 list would require mutable
>>> +        //   access to the cursor or the list. However, the `ArcBorrow=
` holds an immutable borrow
>>> +        //   on the cursor, which in turn holds an immutable borrow on=
 the list, so any such
>>
>> The cursor has a mutable borrow on the list.
>>
>>
>>> +        //   mutable access requires first releasing the immutable bor=
row on the cursor.
>>> +        // * Values in a list never have a `UniqueArc` reference.
>>
>> Is there some type invariant guaranteeing this?
>=20
> The List owns a ListArc reference to the value. It would be unsound
> for there to also be a UniqueArc reference to it.

I think it would be good to add the existence of the `ListArc` as an
explanation.

--=20
Cheers,
Benno