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Date: Mon, 27 May 2024 10:37:34 +0000
To: Alice Ryhl <aliceryhl@google.com>, Miguel Ojeda <ojeda@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
From: Benno Lossin <benno.lossin@proton.me>
Cc: 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 v2 7/9] rust: list: add cursor
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On 06.05.24 11:53, Alice Ryhl wrote:
> The cursor is very similar to the list iterator, but it has one
> important feature that the iterator doesn't: it can be used to remove
> items from the linked list.
>=20
> This feature cannot be added to the iterator because the references you
> get from the iterator are considered borrows of the original list,
> rather than borrows of the iterator. This means that there's no way to
> prevent code like this:
>=20
> let item =3D iter.next();
> iter.remove();
> use(item);
>=20
> If `iter` was a cursor instead of an iterator, then `item` will be
> considered a borrow of `iter`. Since `remove` destroys `iter`, this
> means that the borrow-checker will prevent uses of `item` after the call
> to `remove`.
>=20
> So there is a trade-off between supporting use in traditional for loops,
> and supporting removal of elements as you iterate. Iterators and cursors
> represents two different choices on that spectrum.
>=20
> Rust Binder needs cursors for the list of death notifications that a
> process is currently handling. When userspace tells Binder that it has
> finished processing the death notification, Binder will iterate the list
> to search for the relevant item and remove it.
>=20
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/list.rs | 82 +++++++++++++++++++++++++++++++++++++++++++++++=
++++++
>  1 file changed, 82 insertions(+)

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

---
Cheers,
Benno