Re: [PATCH v10 4/5] rust: rbtree: add cursor

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

On 19.08.24 17:07, Matt Gilbride wrote:
> Add a cursor interface to `RBTree`, supporting the following use cases:
> - Inspect the current node pointed to by the cursor, inspect/move to
>   it's neighbors in sort order (bidirectionally).
> - Mutate the tree itself by removing the current node pointed to by the
>   cursor, or one of its neighbors.
> 
> Add functions to obtain a cursor to the tree by key:
> - The node with the smallest key
> - The node with the largest key
> - The node matching the given key, or the one with the next larger key
> 
> The cursor abstraction is needed by the binder driver to efficiently
> search for nodes and (conditionally) modify them, as well as their
> neighbors [1].
> 
> Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-6-08ba9197f637@google.com/ [1]
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Tested-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Matt Gilbride <mattgilbride@google.com>
> ---
>  rust/kernel/rbtree.rs | 541 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 541 insertions(+)
> 
> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index 6c9a8f0a00e6..754af0db86b5 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -236,6 +236,40 @@ pub fn values(&self) -> impl Iterator<Item = &'_ V> {
>      pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
>          self.iter_mut().map(|(_, v)| v)
>      }
> +
> +    /// Returns a cursor over the tree nodes, starting with the smallest key.
> +    pub fn cursor_front(&mut self) -> Option<Cursor<'_, K, V>> {
> +        let root = addr_of_mut!(self.root);
> +        // SAFETY: `self.root` is always a valid root node
> +        let current = unsafe { bindings::rb_first(root) };
> +        NonNull::new(current).map(|current| {
> +            // INVARIANT:
> +            // - `current` is a valid node in the [`RBTree`] pointed to by `self`.
> +            // - Due to the type signature of this function, the returned [`Cursor`]
> +            //   borrows mutably from `self`.

This second invariant doesn't exist on `Cursor`, probably some stale
comment.

> +            Cursor {
> +                current,
> +                tree: self,
> +            }
> +        })
> +    }
> +
> +    /// Returns a cursor over the tree nodes, starting with the largest key.
> +    pub fn cursor_back(&mut self) -> Option<Cursor<'_, K, V>> {
> +        let root = addr_of_mut!(self.root);
> +        // SAFETY: `self.root` is always a valid root node
> +        let current = unsafe { bindings::rb_last(root) };
> +        NonNull::new(current).map(|current| {
> +            // INVARIANT:
> +            // - `current` is a valid node in the [`RBTree`] pointed to by `self`.
> +            // - Due to the type signature of this function, the returned [`Cursor`]
> +            //   borrows mutably from `self`.

Ditto.

> +            Cursor {
> +                current,
> +                tree: self,
> +            }
> +        })
> +    }
>  }
> 
>  impl<K, V> RBTree<K, V>
> @@ -396,6 +430,75 @@ fn remove_node(&mut self, key: &K) -> Option<RBTreeNode<K, V>> {
>      pub fn remove(&mut self, key: &K) -> Option<V> {
>          self.remove_node(key).map(|node| node.node.value)
>      }
> +
> +    /// Returns a cursor over the tree nodes based on the given key.
> +    ///
> +    /// If the given key exists, the cursor starts there.
> +    /// Otherwise it starts with the first larger key in sort order.
> +    /// If there is no larger key, it returns [`None`].
> +    pub fn cursor_lower_bound(&mut self, key: &K) -> Option<Cursor<'_, K, V>>
> +    where
> +        K: Ord,
> +    {
> +        let mut node = self.root.rb_node;
> +        let mut best_match: Option<NonNull<Node<K, V>>> = None;
> +        while !node.is_null() {
> +            // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
> +            // point to the links field of `Node<K, V>` objects.
> +            let this = unsafe { container_of!(node, Node<K, V>, links) }.cast_mut();
> +            // SAFETY: `this` is a non-null node so it is valid by the type invariants.
> +            let this_key = unsafe { &(*this).key };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let left_child = unsafe { (*node).rb_left };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let right_child = unsafe { (*node).rb_right };
> +            if key == this_key {
> +                return NonNull::new(node).map(|current| {
> +                    // INVARIANT:
> +                    // - `node` is a valid node in the [`RBTree`] pointed to by `self`.
> +                    // - Due to the type signature of this function, the returned [`Cursor`]
> +                    //   borrows mutably from `self`.

Here again the stale invariant.

> +                    Cursor {
> +                        current,
> +                        tree: self,
> +                    }
> +                });

You could do this instead of returning here:

    best_match = NonNull::new(this);
    break;

Then we don't have two places constructing a `Cursor` and can avoid
having to duplicate the `INVARIANT` comment.

> +            } else {
> +                node = if key > this_key {

I don't know how much this cares about performance, but you can avoid
having to do two comparisons by doing `match key.cmp(this_key)`. Maybe
the compiler is already doing that optimization though.

> +                    right_child
> +                } else {
> +                    let is_better_match = match best_match {
> +                        None => true,
> +                        Some(best) => {
> +                            // SAFETY: `best` is a non-null node so it is valid by the type invariants.
> +                            let best_key = unsafe { &(*best.as_ptr()).key };
> +                            best_key > this_key
> +                        }
> +                    };
> +                    if is_better_match {
> +                        best_match = NonNull::new(this);
> +                    }
> +                    left_child
> +                };
> +            }
> +        }
> +
> +        let best = best_match?;
> +
> +        // SAFETY: `best` is a non-null node so it is valid by the type invariants.
> +        let links = unsafe { addr_of_mut!((*best.as_ptr()).links) };
> +
> +        NonNull::new(links).map(|current| {
> +            // INVARIANT:
> +            // - `current` is a valid node in the [`RBTree`] pointed to by `self`.
> +            // - Due to the type signature of this function, the returned [`Cursor`]
> +            //   borrows mutably from `self`.

Stale invariant (more instances below).

---
Cheers,
Benno

> +            Cursor {
> +                current,
> +                tree: self,
> +            }
> +        })
> +    }
>  }
> 
>  impl<K, V> Default for RBTree<K, V> {