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

[Matt Gilbride <mattgilbride@google.com>]

From: Wedson Almeida Filho <wedsonaf@gmail.com>

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@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 | 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 {
+            _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> {
+    _tree: PhantomData<&'a RBTree<K, V>>,
+    next: *mut bindings::rb_node,
+}
+
+// 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