linux-kernel - Re: [PATCH 6/9] rust: list: add iterators

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Date: Thu, 04 Apr 2024 14:52:31 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...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>, Marco Elver <elver@...gle.com>, Kees Cook <keescook@...omium.org>, Coly Li <colyli@...e.de>, Paolo Abeni <pabeni@...hat.com>, Pierre Gondois <pierre.gondois@....com>, Ingo Molnar <mingo@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Wei Yang <richard.weiyang@...il.com>, Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 6/9] rust: list: add iterators

On 04.04.24 16:41, Alice Ryhl wrote:
> On Thu, Apr 4, 2024 at 4:36 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> On 02.04.24 14:17, Alice Ryhl wrote:
>>> +/// the end of the list. The `stop` pointer points at the first value in the same list, or it is
>>> +/// null if the list is empty.
>>> +#[derive(Clone)]
>>> +pub struct Iter<'a, T: ?Sized + ListItem<ID>, const ID: u64 = 0> {
>>> +    current: *mut ListLinksFields,
>>> +    stop: *mut ListLinksFields,
>>> +    _ty: PhantomData<&'a ListArc<T, ID>>,
>>> +}
>>> +
>>> +impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Iterator for Iter<'a, T, ID> {
>>> +    type Item = ArcBorrow<'a, T>;
>>> +
>>> +    fn next(&mut self) -> Option<ArcBorrow<'a, T>> {
>>> +        if self.current.is_null() {
>>> +            return None;
>>> +        }
>>> +
>>> +        let current = self.current;
>>> +
>>> +        // SAFETY: We just checked that `current` is not null, so it is in a list, and hence not
>>> +        // dangling. There's no race because the iterator holds an immutable borrow to the list.
>>
>> This (that the iterator holds an immutable borrow) is not true (there
>> is no `&List` field in `Iter`), but you can make that an invariant
>> instead.
> 
> What I mean is that the borrow-checker will consider the `List` to be
> borrowed by `Iter`. Whether or not there is a real reference or not
> doesn't matter.

Yes, but that is implementation detail of the safe function
`List::iter`, so I think it must also be captured by an invariant.

-- 
Cheers,
Benno