Re: [PATCH v8 4/6] rust: rbtree: add mutable iterator

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

On 27.07.24 22:30, Matt Gilbride wrote:
> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> Add mutable Iterator implementation 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@google.com/ [1]
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Matt Gilbride <mattgilbride@google.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Tested-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/rbtree.rs | 98 ++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 86 insertions(+), 12 deletions(-)
> 
> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index d10074e4ac58..d7514ebadfa8 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -197,8 +197,26 @@ pub fn iter(&self) -> Iter<'_, K, V> {
>          // INVARIANT: `bindings::rb_first` returns a valid pointer to a tree node given a valid pointer to a tree root.

This INVARIANT is out of place, `Iter` doesn't have any INVARIANT any
more.

>          Iter {
>              _tree: PhantomData,
> -            // SAFETY: `self.root` is a valid pointer to the tree root.
> -            next: unsafe { bindings::rb_first(&self.root) },
> +            iter_raw: IterRaw {

This `IterRaw` construction is missing an INVARIANT comment. I think you
can copy paste from below.

> +                // SAFETY: by the invariants, all pointers are valid.
> +                next: unsafe { bindings::rb_first(&self.root) },
> +                _phantom: PhantomData,
> +            },
> +        }
> +    }
> +
> +    /// Returns a mutable iterator over the tree nodes, sorted by key.
> +    pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
> +        IterMut {
> +            _tree: PhantomData,
> +            // INVARIANT:
> +            //   - `self.root` is a valid pointer to a tree root.
> +            //   - `bindings::rb_first` produces a valid pointer to a node given `root` is valid.
> +            iter_raw: IterRaw {
> +                // SAFETY: by the invariants, all pointers are valid.
> +                next: unsafe { bindings::rb_first(&self.root) },

Does this really derive a mutable reference? Ie shouldn't this be:?

    next: unsafe { bindings::rb_first(&mut self.root) },

> +                _phantom: PhantomData,
> +            },
>          }
>      }
> 
> @@ -211,6 +229,11 @@ 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>
> @@ -414,13 +437,9 @@ fn into_iter(self) -> Self::IntoIter {
>  /// An iterator over the nodes of a [`RBTree`].
>  ///
>  /// Instances are created by calling [`RBTree::iter`].
> -///
> -/// # Invariants
> -/// - `self.next` is a valid pointer.
> -/// - `self.next` points to a node stored inside of a valid `RBTree`.
>  pub struct Iter<'a, K, V> {
>      _tree: PhantomData<&'a RBTree<K, V>>,
> -    next: *mut bindings::rb_node,
> +    iter_raw: IterRaw<K, V>,
>  }
> 
>  // SAFETY: The [`Iter`] gives out immutable references to K and V, so it has the same
> @@ -434,21 +453,76 @@ unsafe impl<'a, K: Sync, V: Sync> Sync for Iter<'a, K, V> {}
>  impl<'a, K, V> Iterator for Iter<'a, K, V> {
>      type Item = (&'a K, &'a V);
> 
> +    fn next(&mut self) -> Option<Self::Item> {
> +        self.iter_raw.next().map(|(k, v)|
> +            // SAFETY: Due to `self._tree`, `k` and `v` are valid for the lifetime of `'a`.
> +            unsafe { (&*k, &*v) })

I don't really like the formatting here, can you move the SAFETY one
line upwards? It should format nicely then.

> +    }
> +}
> +
> +impl<'a, K, V> IntoIterator for &'a mut RBTree<K, V> {
> +    type Item = (&'a K, &'a mut V);
> +    type IntoIter = IterMut<'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`].
> +pub struct IterMut<'a, K, V> {
> +    _tree: PhantomData<&'a mut RBTree<K, V>>,
> +    iter_raw: IterRaw<K, V>,
> +}
> +
> +// SAFETY: The [`IterMut`] gives out immutable references to K and mutable references to V, so it has the same
> +// thread safety requirements as mutable references.
> +unsafe impl<'a, K: Send, V: Send> Send for IterMut<'a, K, V> {}

Since we only borrow `K` immutably, would it make sense to have `K:
Sync`?

---
Cheers,
Benno

> +
> +// SAFETY: The [`IterMut`] gives out immutable references to K and mutable references to V, so it has the same
> +// thread safety requirements as mutable references.
> +unsafe impl<'a, K: Sync, V: Sync> Sync for IterMut<'a, K, V> {}
> +
> +impl<'a, K, V> Iterator for IterMut<'a, K, V> {
> +    type Item = (&'a K, &'a mut V);
> +
> +    fn next(&mut self) -> Option<Self::Item> {
> +        self.iter_raw.next().map(|(k, v)|
> +            // SAFETY: Due to `&mut self`, we have exclusive access to `k` and `v`, for the lifetime of `'a`.
> +            unsafe { (&*k, &mut *v) })
> +    }
> +}
> +
> +/// A raw iterator over the nodes of a [`RBTree`].
> +///
> +/// # Invariants
> +/// - `self.next` is a valid pointer.
> +/// - `self.next` points to a node stored inside of a valid `RBTree`.
> +struct IterRaw<K, V> {
> +    next: *mut bindings::rb_node,
> +    _phantom: PhantomData<fn() -> (K, V)>,
> +}
> +
> +impl<K, V> Iterator for IterRaw<K, V> {
> +    type Item = (*mut K, *mut V);
> +
>      fn next(&mut self) -> Option<Self::Item> {
>          if self.next.is_null() {
>              return None;
>          }
> 
> -        // SAFETY: By the type invariant of `Iter`, `self.next` is a valid node in an `RBTree`,
> +        // SAFETY: By the type invariant of `IterRaw`, `self.next` is a valid node in an `RBTree`,
>          // and by the type invariant of `RBTree`, all nodes point to the links field of `Node<K, V>` objects.
> -        let cur = unsafe { container_of!(self.next, Node<K, V>, links) };
> +        let cur: *mut Node<K, V> =
> +            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, &(*cur).value) })
> +        // SAFETY: By the same reasoning above, it is safe to dereference the node.
> +        Some(unsafe { (addr_of_mut!((*cur).key), addr_of_mut!((*cur).value)) })
>      }
>  }
> 
> 
> --
> 2.46.0.rc1.232.g9752f9e123-goog
>