rust: rbtree: add mutable iterator

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>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
Link: https://lore.kernel.org/r/20240822-b4-rbtree-v12-3-014561758a57@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/rbtree.rs

@@ -12,7 +12,7 @@ use core::{
     cmp::{Ord, Ordering},
     marker::PhantomData,
     mem::MaybeUninit,
-    ptr::{addr_of_mut, NonNull},
+    ptr::{addr_of_mut, from_mut, NonNull},
 };
 
 /// A red-black tree with owned nodes.
@@ -194,11 +194,31 @@ impl<K, V> RBTree<K, V> {
 
     /// Returns an iterator over the tree nodes, sorted by key.
     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.
         Iter {
             _tree: PhantomData,
-            // SAFETY: `self.root` is a valid pointer to the tree root.
-            next: unsafe { bindings::rb_first(&self.root) },
+            // 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) },
+                _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(from_mut(&mut self.root)) },
+                _phantom: PhantomData,
+            },
         }
     }
 
@@ -211,6 +231,11 @@ impl<K, V> RBTree<K, V> {
     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 +439,9 @@ impl<'a, K, V> IntoIterator for &'a RBTree<K, V> {
 /// 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 +455,75 @@ 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> {
+        // SAFETY: Due to `self._tree`, `k` and `v` are valid for the lifetime of `'a`.
+        self.iter_raw.next().map(|(k, v)| unsafe { (&*k, &*v) })
+    }
+}
+
+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`] has exclusive access to both `K` and `V`, so it is sufficient to require them to be `Send`.
+// The iterator only gives out immutable references to the keys, but since the iterator has excusive access to those same
+// keys, `Send` is sufficient. `Sync` would be okay, but it is more restrictive to the user.
+unsafe impl<'a, K: Send, V: Send> Send for IterMut<'a, 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: 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 = 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)) })
     }
 }