rust: rbtree: add immutable cursor

Sometimes we may need to iterate over, or find an element in a read
only (or read mostly) red-black tree, and in that case we don't need a
mutable reference to the tree, which we'll however have to take to be
able to use the current (mutable) cursor implementation.

This patch adds a simple immutable cursor implementation to RBTree,
which enables us to use an immutable tree reference. The existing
(fully featured) cursor implementation is renamed to CursorMut,
while retaining its functionality.

The only existing user of the [mutable] cursor for RBTrees (binder) is
updated to match the changes.

Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.se>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251014123339.2492210-1-vitaly.wool@konsulko.se
[ Applied `rustfmt`. Added intra-doc link. Fixed unclosed example.
  Fixed docs description. Fixed typo and other formatting nits.
    - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 197 insertions, 47 deletions

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index b8fe6be6fcc4..4729eb56827a 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -243,34 +243,64 @@ impl<K, V> RBTree<K, V> {
     }
 
     /// Returns a cursor over the tree nodes, starting with the smallest key.
-    pub fn cursor_front(&mut self) -> Option<Cursor<'_, K, V>> {
+    pub fn cursor_front_mut(&mut self) -> Option<CursorMut<'_, K, V>> {
         let root = addr_of_mut!(self.root);
-        // SAFETY: `self.root` is always a valid root node
+        // 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`.
-            Cursor {
+            CursorMut {
                 current,
                 tree: self,
             }
         })
     }
 
+    /// Returns an immutable cursor over the tree nodes, starting with the smallest key.
+    pub fn cursor_front(&self) -> Option<Cursor<'_, K, V>> {
+        let root = &raw const 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`.
+            Cursor {
+                current,
+                _tree: PhantomData,
+            }
+        })
+    }
+
     /// Returns a cursor over the tree nodes, starting with the largest key.
-    pub fn cursor_back(&mut self) -> Option<Cursor<'_, K, V>> {
+    pub fn cursor_back_mut(&mut self) -> Option<CursorMut<'_, K, V>> {
         let root = addr_of_mut!(self.root);
-        // SAFETY: `self.root` is always a valid root node
+        // 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`.
-            Cursor {
+            CursorMut {
                 current,
                 tree: self,
             }
         })
     }
+
+    /// Returns a cursor over the tree nodes, starting with the largest key.
+    pub fn cursor_back(&self) -> Option<Cursor<'_, K, V>> {
+        let root = &raw const 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`.
+            Cursor {
+                current,
+                _tree: PhantomData,
+            }
+        })
+    }
 }
 
 impl<K, V> RBTree<K, V>
@@ -421,12 +451,47 @@ where
     /// 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>>
+    pub fn cursor_lower_bound_mut(&mut self, key: &K) -> Option<CursorMut<'_, K, V>>
+    where
+        K: Ord,
+    {
+        let best = self.find_best_match(key)?;
+
+        NonNull::new(best.as_ptr()).map(|current| {
+            // INVARIANT:
+            // - `current` is a valid node in the [`RBTree`] pointed to by `self`.
+            CursorMut {
+                current,
+                tree: self,
+            }
+        })
+    }
+
+    /// 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(&self, key: &K) -> Option<Cursor<'_, K, V>>
     where
         K: Ord,
     {
+        let best = self.find_best_match(key)?;
+
+        NonNull::new(best.as_ptr()).map(|current| {
+            // INVARIANT:
+            // - `current` is a valid node in the [`RBTree`] pointed to by `self`.
+            Cursor {
+                current,
+                _tree: PhantomData,
+            }
+        })
+    }
+
+    fn find_best_match(&self, key: &K) -> Option<NonNull<bindings::rb_node>> {
         let mut node = self.root.rb_node;
-        let mut best_match: Option<NonNull<Node<K, V>>> = None;
+        let mut best_key: Option<&K> = None;
+        let mut best_links: Option<NonNull<bindings::rb_node>> = 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.
@@ -439,42 +504,28 @@ where
             let right_child = unsafe { (*node).rb_right };
             match key.cmp(this_key) {
                 Ordering::Equal => {
-                    best_match = NonNull::new(this);
+                    // SAFETY: `this` is a non-null node so it is valid by the type invariants.
+                    best_links = Some(unsafe { NonNull::new_unchecked(&mut (*this).links) });
                     break;
                 }
                 Ordering::Greater => {
                     node = right_child;
                 }
                 Ordering::Less => {
-                    let is_better_match = match best_match {
+                    let is_better_match = match best_key {
                         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
-                        }
+                        Some(best) => best > this_key,
                     };
                     if is_better_match {
-                        best_match = NonNull::new(this);
+                        best_key = Some(this_key);
+                        // SAFETY: `this` is a non-null node so it is valid by the type invariants.
+                        best_links = Some(unsafe { NonNull::new_unchecked(&mut (*this).links) });
                     }
                     node = 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`.
-            Cursor {
-                current,
-                tree: self,
-            }
-        })
+        best_links
     }
 }
 
@@ -507,7 +558,7 @@ impl<K, V> Drop for RBTree<K, V> {
     }
 }
 
-/// A bidirectional cursor over the tree nodes, sorted by key.
+/// A bidirectional mutable cursor over the tree nodes, sorted by key.
 ///
 /// # Examples
 ///
@@ -526,7 +577,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
 ///
 /// // Get a cursor to the first element.
-/// let mut cursor = tree.cursor_front().unwrap();
+/// let mut cursor = tree.cursor_front_mut().unwrap();
 /// let mut current = cursor.current();
 /// assert_eq!(current, (&10, &100));
 ///
@@ -564,7 +615,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// tree.try_create_and_insert(20, 200, flags::GFP_KERNEL)?;
 /// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
 ///
-/// let mut cursor = tree.cursor_back().unwrap();
+/// let mut cursor = tree.cursor_back_mut().unwrap();
 /// let current = cursor.current();
 /// assert_eq!(current, (&30, &300));
 ///
@@ -577,7 +628,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// use kernel::rbtree::RBTree;
 ///
 /// let mut tree: RBTree<u16, u16> = RBTree::new();
-/// assert!(tree.cursor_front().is_none());
+/// assert!(tree.cursor_front_mut().is_none());
 ///
 /// # Ok::<(), Error>(())
 /// ```
@@ -628,7 +679,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
 ///
 /// // Retrieve a cursor.
-/// let mut cursor = tree.cursor_front().unwrap();
+/// let mut cursor = tree.cursor_front_mut().unwrap();
 ///
 /// // Get a mutable reference to the current value.
 /// let (k, v) = cursor.current_mut();
@@ -655,7 +706,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
 ///
 /// // Remove the first element.
-/// let mut cursor = tree.cursor_front().unwrap();
+/// let mut cursor = tree.cursor_front_mut().unwrap();
 /// let mut current = cursor.current();
 /// assert_eq!(current, (&10, &100));
 /// cursor = cursor.remove_current().0.unwrap();
@@ -665,7 +716,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// assert_eq!(current, (&20, &200));
 ///
 /// // Get a cursor to the last element, and remove it.
-/// cursor = tree.cursor_back().unwrap();
+/// cursor = tree.cursor_back_mut().unwrap();
 /// current = cursor.current();
 /// assert_eq!(current, (&30, &300));
 ///
@@ -694,7 +745,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
 ///
 /// // Get a cursor to the first element.
-/// let mut cursor = tree.cursor_front().unwrap();
+/// let mut cursor = tree.cursor_front_mut().unwrap();
 /// let mut current = cursor.current();
 /// assert_eq!(current, (&10, &100));
 ///
@@ -702,7 +753,7 @@ impl<K, V> Drop for RBTree<K, V> {
 /// assert!(cursor.remove_prev().is_none());
 ///
 /// // Get a cursor to the last element.
-/// cursor = tree.cursor_back().unwrap();
+/// cursor = tree.cursor_back_mut().unwrap();
 /// current = cursor.current();
 /// assert_eq!(current, (&30, &300));
 ///
@@ -726,18 +777,48 @@ impl<K, V> Drop for RBTree<K, V> {
 ///
 /// # Invariants
 /// - `current` points to a node that is in the same [`RBTree`] as `tree`.
-pub struct Cursor<'a, K, V> {
+pub struct CursorMut<'a, K, V> {
     tree: &'a mut RBTree<K, V>,
     current: NonNull<bindings::rb_node>,
 }
 
-// SAFETY: The [`Cursor`] has exclusive access to both `K` and `V`, so it is sufficient to require them to be `Send`.
-// The cursor only gives out immutable references to the keys, but since it 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 Cursor<'a, K, V> {}
+/// A bidirectional immutable cursor over the tree nodes, sorted by key. This is a simpler
+/// variant of [`CursorMut`] that is basically providing read only access.
+///
+/// # Examples
+///
+/// In the following example, we obtain a cursor to the first element in the tree.
+/// The cursor allows us to iterate bidirectionally over key/value pairs in the tree.
+///
+/// ```
+/// use kernel::{alloc::flags, rbtree::RBTree};
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(10, 100, flags::GFP_KERNEL)?;
+/// tree.try_create_and_insert(20, 200, flags::GFP_KERNEL)?;
+/// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
+///
+/// // Get a cursor to the first element.
+/// let cursor = tree.cursor_front().unwrap();
+/// let current = cursor.current();
+/// assert_eq!(current, (&10, &100));
+///
+/// # Ok::<(), Error>(())
+/// ```
+pub struct Cursor<'a, K, V> {
+    _tree: PhantomData<&'a RBTree<K, V>>,
+    current: NonNull<bindings::rb_node>,
+}
 
-// SAFETY: The [`Cursor`] gives out immutable references to K and mutable references to V,
-// so it has the same thread safety requirements as mutable references.
+// SAFETY: The immutable cursor gives out shared access to `K` and `V` so if `K` and `V` can be
+// shared across threads, then it's safe to share the cursor.
+unsafe impl<'a, K: Sync, V: Sync> Send for Cursor<'a, K, V> {}
+
+// SAFETY: The immutable cursor gives out shared access to `K` and `V` so if `K` and `V` can be
+// shared across threads, then it's safe to share the cursor.
 unsafe impl<'a, K: Sync, V: Sync> Sync for Cursor<'a, K, V> {}
 
 impl<'a, K, V> Cursor<'a, K, V> {
@@ -749,6 +830,75 @@ impl<'a, K, V> Cursor<'a, K, V> {
         unsafe { Self::to_key_value(self.current) }
     }
 
+    /// # Safety
+    ///
+    /// - `node` must be a valid pointer to a node in an [`RBTree`].
+    /// - The caller has immutable access to `node` for the duration of `'b`.
+    unsafe fn to_key_value<'b>(node: NonNull<bindings::rb_node>) -> (&'b K, &'b V) {
+        // 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.as_ptr(), Node<K, V>, links) };
+        // SAFETY: The passed `node` is the current node or a non-null neighbor,
+        // thus `this` is valid by the type invariants.
+        let k = unsafe { &(*this).key };
+        // SAFETY: The passed `node` is the current node or a non-null neighbor,
+        // thus `this` is valid by the type invariants.
+        let v = unsafe { &(*this).value };
+        (k, v)
+    }
+
+    /// Access the previous node without moving the cursor.
+    pub fn peek_prev(&self) -> Option<(&K, &V)> {
+        self.peek(Direction::Prev)
+    }
+
+    /// Access the next node without moving the cursor.
+    pub fn peek_next(&self) -> Option<(&K, &V)> {
+        self.peek(Direction::Next)
+    }
+
+    fn peek(&self, direction: Direction) -> Option<(&K, &V)> {
+        self.get_neighbor_raw(direction).map(|neighbor| {
+            // SAFETY:
+            // - `neighbor` is a valid tree node.
+            // - By the function signature, we have an immutable reference to `self`.
+            unsafe { Self::to_key_value(neighbor) }
+        })
+    }
+
+    fn get_neighbor_raw(&self, direction: Direction) -> Option<NonNull<bindings::rb_node>> {
+        // SAFETY: `self.current` is valid by the type invariants.
+        let neighbor = unsafe {
+            match direction {
+                Direction::Prev => bindings::rb_prev(self.current.as_ptr()),
+                Direction::Next => bindings::rb_next(self.current.as_ptr()),
+            }
+        };
+
+        NonNull::new(neighbor)
+    }
+}
+
+// SAFETY: The [`CursorMut`] has exclusive access to both `K` and `V`, so it is sufficient to
+// require them to be `Send`.
+// The cursor only gives out immutable references to the keys, but since it has exclusive 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 CursorMut<'a, K, V> {}
+
+// SAFETY: The [`CursorMut`] 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 CursorMut<'a, K, V> {}
+
+impl<'a, K, V> CursorMut<'a, K, V> {
+    /// The current node.
+    pub fn current(&self) -> (&K, &V) {
+        // SAFETY:
+        // - `self.current` is a valid node by the type invariants.
+        // - We have an immutable reference by the function signature.
+        unsafe { Self::to_key_value(self.current) }
+    }
+
     /// The current node, with a mutable value
     pub fn current_mut(&mut self) -> (&K, &mut V) {
         // SAFETY:
@@ -920,7 +1070,7 @@ impl<'a, K, V> Cursor<'a, K, V> {
     }
 }
 
-/// Direction for [`Cursor`] operations.
+/// Direction for [`Cursor`] and [`CursorMut`] operations.
 enum Direction {
     /// the node immediately before, in sort order
     Prev,