Re: [PATCH v3 3/5] rust: rbtree: add `RBTreeIteratorMut`

[Benno Lossin]

On 18.04.24 16:15, Matt Gilbride wrote:
> From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
> 
> Add mutable Iterator implementation (`RBTreeIteratorMut`) for `RBTree`,
> allowing iteration over (key, value) pairs in key order. Only values are
> mutable, as mutating keys implies modifying a node's position in the tree.
> 
> Mutable iteration is used by the binder driver during shutdown to
> clean up the tree maintained by the "range allocator" [1].
> 
> Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-6-08ba9197f637@xxxxxxxxxx/ [1]
> Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
> Signed-off-by: Matt Gilbride <mattgilbride@xxxxxxxxxx>
> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Tested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---
>  rust/kernel/rbtree.rs | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
> 
> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index 2f836be7bdbe..50d440c9926d 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -222,6 +222,15 @@ pub fn iter(&self) -> RBTreeIterator<'_, K, V> {
>          }
>      }
> 
> +    /// Returns a mutable iterator over the tree nodes, sorted by key.
> +    pub fn iter_mut(&mut self) -> RBTreeIteratorMut<'_, K, V> {
> +        RBTreeIteratorMut {

This is missing an INVARIANT comment.

> +            _tree: PhantomData,
> +            // SAFETY: `root` is valid as it's embedded in `self` and we have a valid `self`.
> +            next: unsafe { bindings::rb_first(&self.root) },
> +        }
> +    }
> +
>      /// Returns an iterator over the keys of the nodes in the tree, in sorted order.
>      pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
>          self.iter().map(|(k, _)| k)
> @@ -231,6 +240,11 @@ pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
>      pub fn values(&self) -> impl Iterator<Item = &'_ V> {
>          self.iter().map(|(_, v)| v)
>      }
> +
> +    /// Returns a mutable iterator over the values of the nodes in the tree, sorted by key.
> +    pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
> +        self.iter_mut().map(|(_, v)| v)
> +    }
>  }
> 
>  impl<K, V> RBTree<K, V>
> @@ -466,6 +480,56 @@ fn next(&mut self) -> Option<Self::Item> {
>      }
>  }
> 
> +impl<'a, K, V> IntoIterator for &'a mut RBTree<K, V> {
> +    type Item = (&'a K, &'a mut V);
> +    type IntoIter = RBTreeIteratorMut<'a, K, V>;
> +
> +    fn into_iter(self) -> Self::IntoIter {
> +        self.iter_mut()
> +    }
> +}
> +
> +/// A mutable iterator over the nodes of a [`RBTree`].
> +///
> +/// Instances are created by calling [`RBTree::iter_mut`].
> +///
> +/// # Invariants
> +/// - `self.next` is a valid pointer.
> +/// - `self.next` points to a node stored inside of a valid `RBTree`.
> +pub struct RBTreeIteratorMut<'a, K, V> {

I think the names `Iter` and `IterMut` are more natural. That is what
the collections in `std::collections` do. These are in the module
`rbtree`, so you can refer to them as `rbtree::Iter`.

> +    _tree: PhantomData<&'a RBTree<K, V>>,

This should have the type `PhantomData<&'a mut RBTree<K, V>>`.

> +    next: *mut bindings::rb_node,
> +}

You could create a common iterator type, since both `RBTreeIterator` and
`RBTreeIteratorMut` are very similar.

How about a (private) `RawIter`:

     struct RawIter<K, V> {
         next: *mut bindings::rb_node,
         _phantom: PhantomData<fn() -> (K, V)>,
     }

And implement `Iterator` with `Item = (*mut K, *mut V)` for `RawIter`.

Then you can change `Iter` to be:

pub struct Iter<'a, K, V> {
     raw_iter: RawIter<K, V>,
     _tree: PhantomData<&'a RBTree<K, V>>,
}

-- 
Cheers,
Benno

> +
> +// SAFETY: The [`RBTreeIterator`] gives out mutable references to K and V, so it has the same
> +// thread safety requirements as mutable references.
> +unsafe impl<'a, K: Send, V: Send> Send for RBTreeIteratorMut<'a, K, V> {}
> +
> +// SAFETY: The [`RBTreeIterator`] gives out mutable references to K and V, so it has the same
> +// thread safety requirements as mutable references.
> +unsafe impl<'a, K: Sync, V: Sync> Sync for RBTreeIteratorMut<'a, K, V> {}
> +
> +impl<'a, K, V> Iterator for RBTreeIteratorMut<'a, K, V> {
> +    type Item = (&'a K, &'a mut V);
> +
> +    fn next(&mut self) -> Option<Self::Item> {
> +        if self.next.is_null() {
> +            return None;
> +        }
> +
> +        // 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 cur = unsafe { container_of!(self.next, Node<K, V>, links) }.cast_mut();
> +
> +        // SAFETY: `self.next` is a valid tree node by the type invariants.
> +        self.next = unsafe { bindings::rb_next(self.next) };
> +
> +        // SAFETY: By the same reasoning above, it is safe to dereference the node. Additionally,
> +        // it is ok to return a reference to members because the iterator must outlive it.
> +        Some(unsafe { (&(*cur).key, &mut (*cur).value) })
> +    }
> +}
> +
>  /// A memory reservation for a red-black tree node.
>  ///
>  /// It contains the memory needed to hold a node that can be inserted into a red-black tree. One
> 
> --
> 2.44.0.769.g3c40516874-goog
>