Re: [PATCH 7/9] rust: list: add cursor

[Benno Lossin <benno.lossin@proton.me>]

On 02.04.24 14:17, Alice Ryhl wrote:
> diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
> index 892705dd0571..47e52818c7bd 100644
> --- a/rust/kernel/list.rs
> +++ b/rust/kernel/list.rs
> @@ -408,6 +408,20 @@ pub fn push_all_back(&mut self, other: &mut List<T, ID>) {
>          other.first = ptr::null_mut();
>      }
> 
> +    /// Returns a cursor to the first element of the list.
> +    ///
> +    /// If the list is empty, this returns `None`.
> +    pub fn cursor_front(&mut self) -> Option<Cursor<'_, T, ID>> {
> +        if self.first.is_null() {
> +            None
> +        } else {
> +            Some(Cursor {

Missing INVARIANT comment.

> +                current: self.first,
> +                list: self,
> +            })
> +        }
> +    }
> +
>      /// Creates an iterator over the list.
>      pub fn iter(&self) -> Iter<'_, T, ID> {
>          // INVARIANT: If the list is empty, both pointers are null. Otherwise, both pointers point
> @@ -476,6 +490,69 @@ fn next(&mut self) -> Option<ArcBorrow<'a, T>> {
>      }
>  }
> 
> +/// A cursor into a [`List`].
> +///
> +/// # Invariants
> +///
> +/// The `current` pointer points a value in `list`.
> +pub struct Cursor<'a, T: ?Sized + ListItem<ID>, const ID: u64 = 0> {
> +    current: *mut ListLinksFields,
> +    list: &'a mut List<T, ID>,
> +}
> +
> +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 = 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 duration 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 borrow on the cursor.
> +        // * Values in a list never have a `UniqueArc` reference.

Is there some type invariant guaranteeing this?

-- 
Cheers,
Benno

> +        unsafe { ArcBorrow::from_raw(me) }
> +    }
> +
> +    /// Move the cursor to the next element.
> +    pub fn next(self) -> Option<Cursor<'a, T, ID>> {
> +        // SAFETY: The `current` field is always in a list.
> +        let next = unsafe { (*self.current).next };
> +
> +        if next == self.list.first {
> +            None
> +        } else {
> +            Some(Cursor {
> +                current: next,
> +                list: self.list,
> +            })
> +        }
> +    }