[PATCH v2 0/6] Red-black tree abstraction needed by Rust Binder

[PATCH v2 0/6] Red-black tree abstraction needed by Rust Binder

[Matt Gilbride]

This patchset contains the red-black tree abstractions needed by the Rust
implementation of the Binder driver.

Binder driver benefits from O(log n) search/insertion/deletion of
key/value mappings in various places, including `process.rs` and
`range_alloc.rs`.  In `range_alloc.rs`, the ability to store and
search by a generic key type is also useful.

Please see the Rust Binder RFC for usage examples [1]. Note that
the `container_of` macro is currently used only by `rbtree` itself.

Users of "rust: rbtree: add red-black tree implementation backed by the C version"
    [PATCH RFC 03/20] rust_binder: add threading support
    [PATCH RFC 05/20] rust_binder: add nodes and context managers
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add `RBTreeIterator`"
    [PATCH RFC 17/20] rust_binder: add oneway spam detection

Users of "rust: rbtree: add `RBTreeIteratorMut`"
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add `RBTreeCursor`"
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add RBTree::entry"
    Not used in the original RFC, but introduced after further
    code review.  See: https://r.android.com/2849906

The Rust Binder RFC addresses the upstream deprecation of red-black
tree. Quoted here for convenience:

"This RFC uses the kernel's red-black tree for key/value mappings, but we
are aware that the red-black tree is deprecated. We did this to make the
performance comparison more fair, since C binder also uses rbtree for
this. We intend to replace these with XArrays instead. That said, we
don't think that XArray is a good fit for the range allocator, and we
propose to continue using the red-black tree for the range allocator."

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-0-08ba9197f637@google.com/ [1]
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
---
Changes in v2:
- Update documentation link to the C header file
- Use `core::convert::Infallible` in try_reserve_node
- Link to v1: https://lore.kernel.org/r/20240205-b4-rbtree-v1-0-995e3eee38c0@google.com

---
Alice Ryhl (1):
      rust: rbtree: add `RBTree::entry`

Matt Gilbride (1):
      rust: rbtree: add `RBTreeCursor`

Wedson Almeida Filho (4):
      rust: add `container_of!` macro
      rust: rbtree: add red-black tree implementation backed by the C version
      rust: rbtree: add `RBTreeIterator`
      rust: rbtree: add `RBTreeIteratorMut`

 rust/helpers.c        |    7 +
 rust/kernel/lib.rs    |   33 ++
 rust/kernel/rbtree.rs | 1214 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 1254 insertions(+)
---
base-commit: 41bccc98fb7931d63d03f326a746ac4d429c1dd3
change-id: 20231205-b4-rbtree-abb1a016f0a0

Best regards,
-- 
Matt Gilbride <mattgilbride@google.com>

[PATCH v2 1/6] rust: add `container_of!` macro

[Matt Gilbride]

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

This macro is used to obtain a pointer to an entire struct
when given a pointer to a field in that struct.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
---
 rust/kernel/lib.rs | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 7ac39874aeac..c7963efd1318 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -102,3 +102,35 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
     // SAFETY: FFI call.
     unsafe { bindings::BUG() };
 }
+
+/// Produces a pointer to an object from a pointer to one of its fields.
+///
+/// # Safety
+///
+/// The pointer passed to this macro, and the pointer returned by this macro, must both be in
+/// bounds of the same allocation.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::container_of;
+/// struct Test {
+///     a: u64,
+///     b: u32,
+/// }
+///
+/// let test = Test { a: 10, b: 20 };
+/// let b_ptr = &test.b;
+/// // SAFETY: The pointer points at the `b` field of a `Test`, so the resulting pointer will be
+/// // in-bounds of the same allocation as `b_ptr`.
+/// let test_alias = unsafe { container_of!(b_ptr, Test, b) };
+/// assert!(core::ptr::eq(&test, test_alias));
+/// ```
+#[macro_export]
+macro_rules! container_of {
+    ($ptr:expr, $type:ty, $($f:tt)*) => {{
+        let ptr = $ptr as *const _ as *const u8;
+        let offset: usize = ::core::mem::offset_of!($type, $($f)*);
+        ptr.sub(offset) as *const $type
+    }}
+}

-- 
2.44.0.rc0.258.g7320e95886-goog

[PATCH v2 2/6] rust: rbtree: add red-black tree implementation backed by the C version

[Matt Gilbride]

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

The rust rbtree exposes a map-like interface over keys and values,
backed by the kernel red-black tree implementation. Values can be
inserted, deleted, and retrieved from a `RBTree` by key.

This base abstraction is used by binder to store key/value
pairs and perform lookups, for example the patch
"[PATCH RFC 03/20] rust_binder: add threading support"
in the binder RFC [1].

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-3-08ba9197f637@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>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
---
 rust/helpers.c        |   7 +
 rust/kernel/lib.rs    |   1 +
 rust/kernel/rbtree.rs | 404 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 412 insertions(+)

diff --git a/rust/helpers.c b/rust/helpers.c
index 70e59efd92bc..56ec79e823df 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -157,6 +157,13 @@ void rust_helper_init_work_with_key(struct work_struct *work, work_func_t func,
 }
 EXPORT_SYMBOL_GPL(rust_helper_init_work_with_key);
 
+void rust_helper_rb_link_node(struct rb_node *node, struct rb_node *parent,
+			      struct rb_node **rb_link)
+{
+	rb_link_node(node, parent, rb_link);
+}
+EXPORT_SYMBOL_GPL(rust_helper_rb_link_node);
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index c7963efd1318..eb84ffba1831 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -43,6 +43,7 @@
 pub mod net;
 pub mod prelude;
 pub mod print;
+pub mod rbtree;
 mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;
diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
new file mode 100644
index 000000000000..a72e9f57e660
--- /dev/null
+++ b/rust/kernel/rbtree.rs
@@ -0,0 +1,404 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Red-black trees.
+//!
+//! C header: [`include/linux/rbtree.h`](srctree/include/linux/rbtree.h)
+//!
+//! Reference: <https://www.kernel.org/doc/html/latest/core-api/rbtree.html>
+
+use crate::{bindings, error::Result, prelude::*};
+use alloc::boxed::Box;
+use core::{
+    cmp::{Ord, Ordering},
+    convert::Infallible,
+    marker::PhantomData,
+    mem::MaybeUninit,
+    ptr::{addr_of_mut, NonNull},
+};
+
+struct Node<K, V> {
+    links: bindings::rb_node,
+    key: K,
+    value: V,
+}
+
+/// A red-black tree with owned nodes.
+///
+/// It is backed by the kernel C red-black trees.
+///
+/// # Invariants
+///
+/// Non-null parent/children pointers stored in instances of the `rb_node` C struct are always
+/// valid, and pointing to a field of our internal representation of a node.
+///
+/// # Examples
+///
+/// In the example below we do several operations on a tree. We note that insertions may fail if
+/// the system is out of memory.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(30, 300)?;
+///
+/// // Check the nodes we just inserted.
+/// {
+///     assert_eq!(tree.get(&10).unwrap(), &100);
+///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(tree.get(&30).unwrap(), &300);
+/// }
+///
+/// // Replace one of the elements.
+/// tree.try_create_and_insert(10, 1000)?;
+///
+/// // Check that the tree reflects the replacement.
+/// {
+///     assert_eq!(tree.get(&10).unwrap(), &1000);
+///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(tree.get(&30).unwrap(), &300);
+/// }
+///
+/// // Change the value of one of the elements.
+/// *tree.get_mut(&30).unwrap() = 3000;
+///
+/// // Check that the tree reflects the update.
+/// {
+///     assert_eq!(tree.get(&10).unwrap(), &1000);
+///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(tree.get(&30).unwrap(), &3000);
+/// }
+///
+/// // Remove an element.
+/// tree.remove(&10);
+///
+/// // Check that the tree reflects the removal.
+/// {
+///     assert_eq!(tree.get(&10), None);
+///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(tree.get(&30).unwrap(), &3000);
+/// }
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// In the example below, we first allocate a node, acquire a spinlock, then insert the node into
+/// the tree. This is useful when the insertion context does not allow sleeping, for example, when
+/// holding a spinlock.
+///
+/// ```
+/// use kernel::{rbtree::RBTree, sync::SpinLock};
+///
+/// fn insert_test(tree: &SpinLock<RBTree<u32, u32>>) -> Result {
+///     // Pre-allocate node. This may fail (as it allocates memory).
+///     let node = RBTree::try_allocate_node(10, 100)?;
+///
+///     // Insert node while holding the lock. It is guaranteed to succeed with no allocation
+///     // attempts.
+///     let mut guard = tree.lock();
+///     guard.insert(node);
+///     Ok(())
+/// }
+/// ```
+///
+/// In the example below, we reuse an existing node allocation from an element we removed.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(30, 300)?;
+///
+/// // Check the nodes we just inserted.
+/// {
+///     assert_eq!(tree.get(&10).unwrap(), &100);
+///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(tree.get(&30).unwrap(), &300);
+/// }
+///
+/// // Remove a node, getting back ownership of it.
+/// let existing = tree.remove_node(&30).unwrap();
+///
+/// // Check that the tree reflects the removal.
+/// {
+///     assert_eq!(tree.get(&10).unwrap(), &100);
+///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(tree.get(&30), None);
+/// }
+///
+/// // Turn the node into a reservation so that we can reuse it with a different key/value.
+/// let reservation = existing.into_reservation();
+///
+/// // Insert a new node into the tree, reusing the previous allocation. This is guaranteed to
+/// // succeed (no memory allocations).
+/// tree.insert(reservation.into_node(15, 150));
+///
+/// // Check that the tree reflect the new insertion.
+/// {
+///     assert_eq!(tree.get(&10).unwrap(), &100);
+///     assert_eq!(tree.get(&15).unwrap(), &150);
+///     assert_eq!(tree.get(&20).unwrap(), &200);
+/// }
+///
+/// # Ok::<(), Error>(())
+/// ```
+pub struct RBTree<K, V> {
+    root: bindings::rb_root,
+    _p: PhantomData<Node<K, V>>,
+}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Send condition as would be used for a struct with K and V fields.
+unsafe impl<K: Send, V: Send> Send for RBTree<K, V> {}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
+unsafe impl<K: Sync, V: Sync> Sync for RBTree<K, V> {}
+
+impl<K, V> RBTree<K, V> {
+    /// Creates a new and empty tree.
+    pub fn new() -> Self {
+        Self {
+            // INVARIANT: There are no nodes in the tree, so the invariant holds vacuously.
+            root: bindings::rb_root::default(),
+            _p: PhantomData,
+        }
+    }
+
+    /// Allocates memory for a node to be eventually initialised and inserted into the tree via a
+    /// call to [`RBTree::insert`].
+    pub fn try_reserve_node() -> Result<RBTreeNodeReservation<K, V>> {
+        Ok(RBTreeNodeReservation {
+            node: Box::init::<Infallible>(crate::init::uninit())?,
+        })
+    }
+
+    /// Allocates and initialises a node that can be inserted into the tree via
+    /// [`RBTree::insert`].
+    pub fn try_allocate_node(key: K, value: V) -> Result<RBTreeNode<K, V>> {
+        Ok(Self::try_reserve_node()?.into_node(key, value))
+    }
+}
+
+impl<K, V> RBTree<K, V>
+where
+    K: Ord,
+{
+    /// Tries to insert a new value into the tree.
+    ///
+    /// It overwrites a node if one already exists with the same key and returns it (containing the
+    /// key/value pair). Returns [`None`] if a node with the same key didn't already exist.
+    ///
+    /// Returns an error if it cannot allocate memory for the new node.
+    pub fn try_create_and_insert(&mut self, key: K, value: V) -> Result<Option<RBTreeNode<K, V>>> {
+        Ok(self.insert(Self::try_allocate_node(key, value)?))
+    }
+
+    /// Inserts a new node into the tree.
+    ///
+    /// It overwrites a node if one already exists with the same key and returns it (containing the
+    /// key/value pair). Returns [`None`] if a node with the same key didn't already exist.
+    ///
+    /// This function always succeeds.
+    pub fn insert(&mut self, node: RBTreeNode<K, V>) -> Option<RBTreeNode<K, V>> {
+        let RBTreeNode { node } = node;
+        let node = Box::into_raw(node);
+        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
+        // the node is removed or replaced.
+        let node_links = unsafe { addr_of_mut!((*node).links) };
+        let mut new_link: &mut *mut bindings::rb_node = &mut self.root.rb_node;
+        let mut parent = core::ptr::null_mut();
+        while !new_link.is_null() {
+            // SAFETY: All links fields we create are in a `Node<K, V>`.
+            let this = unsafe { crate::container_of!(*new_link, Node<K, V>, links) };
+
+            parent = *new_link;
+
+            // SAFETY: `this` is a non-null node so it is valid by the type invariants. `node` is
+            // valid until the node is removed.
+            match unsafe { (*node).key.cmp(&(*this).key) } {
+                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
+                Ordering::Less => new_link = unsafe { &mut (*parent).rb_left },
+                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
+                Ordering::Greater => new_link = unsafe { &mut (*parent).rb_right },
+                Ordering::Equal => {
+                    // INVARIANT: We are replacing an existing node with a new one, which is valid.
+                    // It remains valid because we "forgot" it with `Box::into_raw`.
+                    // SAFETY: All pointers are non-null and valid (parent, despite the name, really
+                    // is the node we're replacing).
+                    unsafe { bindings::rb_replace_node(parent, node_links, &mut self.root) };
+
+                    // INVARIANT: The node is being returned and the caller may free it, however,
+                    // it was removed from the tree. So the invariants still hold.
+                    return Some(RBTreeNode {
+                        // SAFETY: `this` was a node in the tree, so it is valid.
+                        node: unsafe { Box::from_raw(this as _) },
+                    });
+                }
+            }
+        }
+
+        // INVARIANT: We are linking in a new node, which is valid. It remains valid because we
+        // "forgot" it with `Box::into_raw`.
+        // SAFETY: All pointers are non-null and valid (`*new_link` is null, but `new_link` is a
+        // mutable reference).
+        unsafe { bindings::rb_link_node(node_links, parent, new_link) };
+
+        // SAFETY: All pointers are valid. `node` has just been inserted into the tree.
+        unsafe { bindings::rb_insert_color(node_links, &mut self.root) };
+        None
+    }
+
+    /// Returns a node with the given key, if one exists.
+    fn find(&self, key: &K) -> Option<NonNull<Node<K, V>>> {
+        let mut node = self.root.rb_node;
+        while !node.is_null() {
+            // SAFETY: All links fields we create are in a `Node<K, V>`.
+            let this = unsafe { crate::container_of!(node, Node<K, V>, links) };
+            // SAFETY: `this` is a non-null node so it is valid by the type invariants.
+            node = match key.cmp(unsafe { &(*this).key }) {
+                // SAFETY: `node` is a non-null node so it is valid by the type invariants.
+                Ordering::Less => unsafe { (*node).rb_left },
+                // SAFETY: `node` is a non-null node so it is valid by the type invariants.
+                Ordering::Greater => unsafe { (*node).rb_right },
+                Ordering::Equal => return NonNull::new(this as _),
+            }
+        }
+        None
+    }
+
+    /// Returns a reference to the value corresponding to the key.
+    pub fn get(&self, key: &K) -> Option<&V> {
+        // SAFETY: The `find` return value is a node in the tree, so it is valid.
+        self.find(key).map(|node| unsafe { &node.as_ref().value })
+    }
+
+    /// Returns a mutable reference to the value corresponding to the key.
+    pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
+        // SAFETY: The `find` return value is a node in the tree, so it is valid.
+        self.find(key)
+            .map(|mut node| unsafe { &mut node.as_mut().value })
+    }
+
+    /// Removes the node with the given key from the tree.
+    ///
+    /// It returns the node that was removed if one exists, or [`None`] otherwise.
+    fn remove_node(&mut self, key: &K) -> Option<RBTreeNode<K, V>> {
+        let mut node = self.find(key)?;
+
+        // SAFETY: The `find` return value is a node in the tree, so it is valid.
+        unsafe { bindings::rb_erase(&mut node.as_mut().links, &mut self.root) };
+
+        // INVARIANT: The node is being returned and the caller may free it, however, it was
+        // removed from the tree. So the invariants still hold.
+        Some(RBTreeNode {
+            // SAFETY: The `find` return value was a node in the tree, so it is valid.
+            node: unsafe { Box::from_raw(node.as_ptr()) },
+        })
+    }
+
+    /// Removes the node with the given key from the tree.
+    ///
+    /// It returns the value that was removed if one exists, or [`None`] otherwise.
+    pub fn remove(&mut self, key: &K) -> Option<V> {
+        let node = self.remove_node(key)?;
+        let RBTreeNode { node } = node;
+        let Node {
+            links: _,
+            key: _,
+            value,
+        } = *node;
+        Some(value)
+    }
+}
+
+impl<K, V> Default for RBTree<K, V> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K, V> Drop for RBTree<K, V> {
+    fn drop(&mut self) {
+        // SAFETY: `root` is valid as it's embedded in `self` and we have a valid `self`.
+        let mut next = unsafe { bindings::rb_first_postorder(&self.root) };
+
+        // INVARIANT: The loop invariant is that all tree nodes from `next` in postorder are valid.
+        while !next.is_null() {
+            // SAFETY: All links fields we create are in a `Node<K, V>`.
+            let this = unsafe { crate::container_of!(next, Node<K, V>, links) };
+
+            // Find out what the next node is before disposing of the current one.
+            // SAFETY: `next` and all nodes in postorder are still valid.
+            next = unsafe { bindings::rb_next_postorder(next) };
+
+            // INVARIANT: This is the destructor, so we break the type invariant during clean-up,
+            // but it is not observable. The loop invariant is still maintained.
+            // SAFETY: `this` is valid per the loop invariant.
+            unsafe { drop(Box::from_raw(this as *mut Node<K, V>)) };
+        }
+    }
+}
+
+/// 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
+/// can be obtained by directly allocating it ([`RBTree::try_reserve_node`]).
+pub struct RBTreeNodeReservation<K, V> {
+    node: Box<MaybeUninit<Node<K, V>>>,
+}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Send condition as would be used for a struct with K and V fields.
+unsafe impl<K: Send, V: Send> Send for RBTreeNodeReservation<K, V> {}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
+unsafe impl<K: Sync, V: Sync> Sync for RBTreeNodeReservation<K, V> {}
+
+impl<K, V> RBTreeNodeReservation<K, V> {
+    /// Initialises a node reservation.
+    ///
+    /// It then becomes an [`RBTreeNode`] that can be inserted into a tree.
+    pub fn into_node(mut self, key: K, value: V) -> RBTreeNode<K, V> {
+        let node_ptr = self.node.as_mut_ptr();
+        // SAFETY: `node_ptr` is valid, and so are its fields.
+        unsafe { addr_of_mut!((*node_ptr).links).write(bindings::rb_node::default()) };
+        // SAFETY: `node_ptr` is valid, and so are its fields.
+        unsafe { addr_of_mut!((*node_ptr).key).write(key) };
+        // SAFETY: `node_ptr` is valid, and so are its fields.
+        unsafe { addr_of_mut!((*node_ptr).value).write(value) };
+        let raw = Box::into_raw(self.node);
+        RBTreeNode {
+            // SAFETY: The pointer came from a `MaybeUninit<Node>` whose fields have all been
+            // initialised. Additionally, it has the same layout as `Node`.
+            node: unsafe { Box::from_raw(raw as _) },
+        }
+    }
+}
+
+/// A red-black tree node.
+///
+/// The node is fully initialised (with key and value) and can be inserted into a tree without any
+/// extra allocations or failure paths.
+pub struct RBTreeNode<K, V> {
+    node: Box<Node<K, V>>,
+}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Send condition as would be used for a struct with K and V fields.
+unsafe impl<K: Send, V: Send> Send for RBTreeNode<K, V> {}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
+unsafe impl<K: Sync, V: Sync> Sync for RBTreeNode<K, V> {}

-- 
2.44.0.rc0.258.g7320e95886-goog

[PATCH v2 3/6] rust: rbtree: add `RBTreeIterator`

[Matt Gilbride]

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

- Add Iterator implementation (`RBTreeIterator`) for `RBTree`, allowing
  iteration over (key, value) pairs in key order.
- Add individual `keys()` and `values()` functions to iterate over keys
  or values alone.
- Update doctests to use iteration instead of explicitly getting items.

Iteration is needed by the binder driver to enumerate all values in a
tree for oneway spam detection [1].

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-17-08ba9197f637@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>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
---
 rust/kernel/rbtree.rs | 125 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 107 insertions(+), 18 deletions(-)

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index a72e9f57e660..b1faac831cfc 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -54,14 +54,30 @@ struct Node<K, V> {
 ///     assert_eq!(tree.get(&30).unwrap(), &300);
 /// }
 ///
+/// // Iterate over the nodes we just inserted.
+/// {
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &300));
+///     assert!(iter.next().is_none());
+/// }
+///
+/// // Print all elements.
+/// for (key, value) in &tree {
+///     pr_info!("{} = {}\n", key, value);
+/// }
+///
 /// // Replace one of the elements.
 /// tree.try_create_and_insert(10, 1000)?;
 ///
 /// // Check that the tree reflects the replacement.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &1000);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &300);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &1000));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &300));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Change the value of one of the elements.
@@ -69,9 +85,11 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflects the update.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &1000);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &3000);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &1000));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &3000));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Remove an element.
@@ -79,9 +97,10 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflects the removal.
 /// {
-///     assert_eq!(tree.get(&10), None);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &3000);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &3000));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// # Ok::<(), Error>(())
@@ -121,9 +140,11 @@ struct Node<K, V> {
 ///
 /// // Check the nodes we just inserted.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &300);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &300));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Remove a node, getting back ownership of it.
@@ -131,9 +152,10 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflects the removal.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30), None);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Turn the node into a reservation so that we can reuse it with a different key/value.
@@ -145,9 +167,11 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflect the new insertion.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
-///     assert_eq!(tree.get(&15).unwrap(), &150);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
+///     assert_eq!(iter.next().unwrap(), (&15, &150));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// # Ok::<(), Error>(())
@@ -188,6 +212,25 @@ pub fn try_reserve_node() -> Result<RBTreeNodeReservation<K, V>> {
     pub fn try_allocate_node(key: K, value: V) -> Result<RBTreeNode<K, V>> {
         Ok(Self::try_reserve_node()?.into_node(key, value))
     }
+
+    /// Returns an iterator over the tree nodes, sorted by key.
+    pub fn iter(&self) -> RBTreeIterator<'_, K, V> {
+        RBTreeIterator {
+            _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)
+    }
+
+    /// Returns an iterator over the values of the nodes in the tree, sorted by key.
+    pub fn values(&self) -> impl Iterator<Item = &'_ V> {
+        self.iter().map(|(_, v)| v)
+    }
 }
 
 impl<K, V> RBTree<K, V>
@@ -350,6 +393,52 @@ fn drop(&mut self) {
     }
 }
 
+impl<'a, K, V> IntoIterator for &'a RBTree<K, V> {
+    type Item = (&'a K, &'a V);
+    type IntoIter = RBTreeIterator<'a, K, V>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+/// An iterator over the nodes of a [`RBTree`].
+///
+/// Instances are created by calling [`RBTree::iter`].
+pub struct RBTreeIterator<'a, K, V> {
+    _tree: PhantomData<&'a RBTree<K, V>>,
+    next: *mut bindings::rb_node,
+}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Send condition as would be used for a struct with K and V fields.
+unsafe impl<'a, K: Send, V: Send> Send for RBTreeIterator<'a, K, V> {}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
+unsafe impl<'a, K: Sync, V: Sync> Sync for RBTreeIterator<'a, K, V> {}
+
+impl<'a, K, V> Iterator for RBTreeIterator<'a, K, V> {
+    type Item = (&'a K, &'a V);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.next.is_null() {
+            return None;
+        }
+
+        // SAFETY: All links fields we create are in a `Node<K, V>`.
+        let cur = unsafe { crate::container_of!(self.next, Node<K, V>, links) };
+
+        // SAFETY: The reference to the tree used to create the iterator outlives the iterator, so
+        // the tree cannot change. By the tree invariant, all nodes are valid.
+        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) })
+    }
+}
+
 /// 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.rc0.258.g7320e95886-goog

[PATCH v2 4/6] rust: rbtree: add `RBTreeIteratorMut`

[Matt Gilbride]

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 | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index b1faac831cfc..ccf74e0dc3ec 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>
@@ -439,6 +453,53 @@ 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`].
+pub struct RBTreeIteratorMut<'a, K, V> {
+    _tree: PhantomData<&'a RBTree<K, V>>,
+    next: *mut bindings::rb_node,
+}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Send condition as would be used for a struct with K and V fields.
+unsafe impl<'a, K: Send, V: Send> Send for RBTreeIteratorMut<'a, K, V> {}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
+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: All links fields we create are in a `Node<K, V>`.
+        let cur = unsafe { crate::container_of!(self.next, Node<K, V>, links) }.cast_mut();
+
+        // SAFETY: The reference to the tree used to create the iterator outlives the iterator, so
+        // the tree cannot change (except for the value of previous nodes, but those don't affect
+        // the iteration process). By the tree invariant, all nodes are valid.
+        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.rc0.258.g7320e95886-goog

[PATCH v2 5/6] rust: rbtree: add `RBTreeCursor`

[Matt Gilbride]

Add a cursor interface to `RBTree`, supporting the following use cases:
- Inspect the current node pointed to by the cursor, inspect/move to
  it's neighbors in sort order (bidirectionally).
- Mutate the tree itself by removing the current node pointed to by the
  cursor, or one of its neighbors.

Add functions to obtain a cursor to the tree by key:
- The node with the smallest key
- The node with the largest key
- The node matching the given key, or the one with the next larger key

The cursor abstraction is needed by the binder driver to efficiently
search for nodes and (conditionally) modify them, as well as their
neighbors [1].

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-6-08ba9197f637@google.com/ [1]
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
---
 rust/kernel/rbtree.rs | 512 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 512 insertions(+)

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index ccf74e0dc3ec..6c3b10e46517 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -245,6 +245,36 @@ pub fn values(&self) -> impl Iterator<Item = &'_ V> {
     pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
         self.iter_mut().map(|(_, v)| v)
     }
+
+    /// Returns a cursor over the tree nodes, starting with the smallest key.
+    pub fn cursor_front(&mut self) -> Option<RBTreeCursor<'_, K, V>> {
+        let root = addr_of_mut!(self.root);
+        // SAFETY: `self.root` is always a valid root node
+        let current = unsafe { bindings::rb_first(root) };
+        if current.is_null() {
+            return None;
+        }
+        Some(RBTreeCursor {
+            _tree: PhantomData,
+            root,
+            current,
+        })
+    }
+
+    /// Returns a cursor over the tree nodes, starting with the largest key.
+    pub fn cursor_back(&mut self) -> Option<RBTreeCursor<'_, K, V>> {
+        let root = addr_of_mut!(self.root);
+        // SAFETY: `self.root` is always a valid root node
+        let current = unsafe { bindings::rb_last(root) };
+        if current.is_null() {
+            return None;
+        }
+        Some(RBTreeCursor {
+            _tree: PhantomData,
+            root,
+            current,
+        })
+    }
 }
 
 impl<K, V> RBTree<K, V>
@@ -377,6 +407,59 @@ pub fn remove(&mut self, key: &K) -> Option<V> {
         } = *node;
         Some(value)
     }
+
+    /// 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(&mut self, key: &K) -> Option<RBTreeCursor<'_, K, V>>
+    where
+        K: Ord,
+    {
+        let mut node = self.root.rb_node;
+        let mut best_match: Option<NonNull<Node<K, V>>> = None;
+        while !node.is_null() {
+            // SAFETY: All links fields we create are in a `Node<K, V>`.
+            let this = unsafe { crate::container_of!(node, Node<K, V>, links) }.cast_mut();
+            // SAFETY: `this` is a non-null node so it is valid by the type invariants.
+            let this_key = unsafe { &(*this).key };
+            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
+            let left_child = unsafe { (*node).rb_left };
+            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
+            let right_child = unsafe { (*node).rb_right };
+            if key == this_key {
+                return Some(RBTreeCursor {
+                    _tree: PhantomData,
+                    root: addr_of_mut!(self.root),
+                    current: node,
+                });
+            } else {
+                node = if key > this_key {
+                    right_child
+                } else {
+                    let is_better_match = match best_match {
+                        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
+                        }
+                    };
+                    if is_better_match {
+                        best_match = NonNull::new(this);
+                    }
+                    left_child
+                }
+            };
+        }
+        best_match.map(|best| RBTreeCursor {
+            _tree: PhantomData,
+            root: addr_of_mut!(self.root),
+            // SAFETY: `best` is a non-null node so it is valid by the type invariants.
+            current: unsafe { addr_of_mut!((*best.as_ptr()).links) },
+        })
+    }
 }
 
 impl<K, V> Default for RBTree<K, V> {
@@ -407,6 +490,435 @@ fn drop(&mut self) {
     }
 }
 
+/// A bidirectional cursor over the tree nodes, sorted by key.
+///
+/// # Invariants
+///
+/// In instance of `RBTreeCursor` is only acquired from [`RBTree`].
+/// A reference to the tree used to create the cursor outlives the cursor, so
+/// the tree cannot change. By the tree invariant, all nodes are valid.
+///
+/// # 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::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(30, 300)?;
+///
+/// // Get a cursor to the first element.
+/// let mut cursor = tree.cursor_front().unwrap();
+/// let mut current = cursor.current();
+/// assert_eq!(current, (&10, &100));
+///
+/// // Move the cursor, updating it to the 2nd element.
+/// cursor = cursor.move_next().unwrap();
+/// current = cursor.current();
+/// assert_eq!(current, (&20, &200));
+///
+/// // Peek at the next element without impacting the cursor.
+/// let next = cursor.peek_next().unwrap();
+/// assert_eq!(next, (&30, &300));
+/// current = cursor.current();
+/// assert_eq!(current, (&20, &200));
+///
+/// // Moving past the last element causes the cursor to return [`None`].
+/// cursor = cursor.move_next().unwrap();
+/// current = cursor.current();
+/// assert_eq!(current, (&30, &300));
+/// let cursor = cursor.move_next();
+/// assert!(cursor.is_none());
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// A cursor can also be obtained at the last element in the tree.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(30, 300)?;
+///
+/// let mut cursor = tree.cursor_back().unwrap();
+/// let current = cursor.current();
+/// assert_eq!(current, (&30, &300));
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// Obtaining a cursor returns [`None`] if the tree is empty.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// let mut tree: RBTree<u16, u16> = RBTree::new();
+/// assert!(tree.cursor_front().is_none());
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// [`RBTree::cursor_lower_bound`] can be used to start at an arbitrary node in the tree.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert five elements.
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(30, 300)?;
+/// tree.try_create_and_insert(40, 400)?;
+/// tree.try_create_and_insert(50, 500)?;
+///
+/// // If the provided key exists, a cursor to that key is returned.
+/// let cursor = tree.cursor_lower_bound(&20).unwrap();
+/// let current = cursor.current();
+/// assert_eq!(current, (&20, &200));
+///
+/// // If the provided key doesn't exist, a cursor to the first larger element in sort order is returned.
+/// let cursor = tree.cursor_lower_bound(&25).unwrap();
+/// let current = cursor.current();
+/// assert_eq!(current, (&30, &300));
+///
+/// // If there is no larger key, [`None`] is returned.
+/// let cursor = tree.cursor_lower_bound(&55);
+/// assert!(cursor.is_none());
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// The cursor allows mutation of values in the tree.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(30, 300)?;
+///
+/// // Retrieve a cursor.
+/// let mut cursor = tree.cursor_front().unwrap();
+///
+/// // Get a mutable reference to the current value.
+/// let (k, v) = cursor.current_mut();
+/// *v = 1000;
+///
+/// // The updated value is reflected in the tree.
+/// let updated = tree.get(&10).unwrap();
+/// assert_eq!(updated, &1000);
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// It also allows node removal. The following examples demonstrate the behavior of removing the current node.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(30, 300)?;
+///
+/// // Remove the first element.
+/// let mut cursor = tree.cursor_front().unwrap();
+/// let mut current = cursor.current();
+/// assert_eq!(current, (&10, &100));
+/// cursor = cursor.remove_current().unwrap();
+///
+/// // If a node exists after the current element, it is returned.
+/// current = cursor.current();
+/// assert_eq!(current, (&20, &200));
+///
+/// // Get a cursor to the last element, and remove it.
+/// cursor = tree.cursor_back().unwrap();
+/// current = cursor.current();
+/// assert_eq!(current, (&30, &300));
+///
+/// // Since there is no next node, the previous node is returned.
+/// cursor = cursor.remove_current().unwrap();
+/// current = cursor.current();
+/// assert_eq!(current, (&20, &200));
+///
+/// // Removing the last element in the tree returns [`None`].
+/// assert!(cursor.remove_current().is_none());
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// Nodes adjacent to the current node can also be removed.
+///
+/// ```
+/// use kernel::rbtree::RBTree;
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(10, 100)?;
+/// tree.try_create_and_insert(20, 200)?;
+/// tree.try_create_and_insert(30, 300)?;
+///
+/// // Get a cursor to the first element.
+/// let mut cursor = tree.cursor_front().unwrap();
+/// let mut current = cursor.current();
+/// assert_eq!(current, (&10, &100));
+///
+/// // Calling `remove_prev` from the first element returns [`None`].
+/// assert!(cursor.remove_prev().is_none());
+///
+/// // Get a cursor to the last element.
+/// cursor = tree.cursor_back().unwrap();
+/// current = cursor.current();
+/// assert_eq!(current, (&30, &300));
+///
+/// // Calling `remove_prev` removes and returns the middle element.
+/// assert_eq!(cursor.remove_prev().unwrap(), (20, 200));
+///
+/// // Calling `remove_next` from the last element returns [`None`].
+/// assert!(cursor.remove_next().is_none());
+///
+/// // Move to the first element
+/// cursor = cursor.move_prev().unwrap();
+/// current = cursor.current();
+/// assert_eq!(current, (&10, &100));
+///
+/// // Calling `remove_next` removes and returns the last element.
+/// assert_eq!(cursor.remove_next().unwrap(), (30, 300));
+///
+/// # Ok::<(), Error>(())
+/// ```
+pub struct RBTreeCursor<'a, K, V> {
+    _tree: PhantomData<&'a RBTree<K, V>>,
+    root: *mut bindings::rb_root,
+    current: *mut bindings::rb_node,
+}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Send condition as would be used for a struct with K and V fields.
+unsafe impl<'a, K: Send, V: Send> Send for RBTreeCursor<'a, K, V> {}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
+unsafe impl<'a, K: Sync, V: Sync> Sync for RBTreeCursor<'a, K, V> {}
+
+impl<'a, K, V> RBTreeCursor<'a, K, V> {
+    /// The current node
+    pub fn current(&self) -> (&K, &V) {
+        Self::to_key_value(self.current)
+    }
+
+    /// The current node, with a mutable value
+    pub fn current_mut(&mut self) -> (&K, &mut V) {
+        Self::to_key_value_mut(self.current)
+    }
+
+    /// Remove the current node from the tree.
+    ///
+    /// Returns a cursor to the next node, if it exists,
+    /// else the previous node. Returns [`None`] if the tree
+    /// becomes empty.
+    pub fn remove_current(mut self) -> Option<Self> {
+        let prev = self.get_neighbor_raw(Direction::Prev);
+        let next = self.get_neighbor_raw(Direction::Next);
+        // SAFETY: All links fields we create are in a `Node<K, V>`.
+        let this = unsafe { crate::container_of!(self.current, Node<K, V>, links) }.cast_mut();
+        // SAFETY: The reference to the tree used to create the cursor outlives the cursor, so
+        // the tree cannot change. By the tree invariant, all nodes are valid.
+        unsafe { bindings::rb_erase(&mut (*this).links, self.root) };
+
+        let current = match (prev, next) {
+            (_, Some(next)) => next,
+            (Some(prev), None) => prev,
+            (None, None) => {
+                return None;
+            }
+        };
+
+        Some(Self {
+            current,
+            _tree: self._tree,
+            root: self.root,
+        })
+    }
+
+    /// Remove the previous node, returning it if it exists.
+    pub fn remove_prev(&mut self) -> Option<(K, V)> {
+        self.remove_neighbor(Direction::Prev)
+    }
+
+    /// Remove the next node, returning it if it exists.
+    pub fn remove_next(&mut self) -> Option<(K, V)> {
+        self.remove_neighbor(Direction::Next)
+    }
+
+    fn remove_neighbor(&mut self, direction: Direction) -> Option<(K, V)> {
+        if let Some(neighbor) = self.get_neighbor_raw(direction) {
+            // SAFETY: All links fields we create are in a `Node<K, V>`.
+            let this = unsafe { crate::container_of!(neighbor, Node<K, V>, links) }.cast_mut();
+            // SAFETY: The reference to the tree used to create the cursor outlives the cursor, so
+            // the tree cannot change. By the tree invariant, all nodes are valid.
+            unsafe { bindings::rb_erase(&mut (*this).links, self.root) };
+            return Some(Self::to_key_value_owned(neighbor));
+        }
+        None
+    }
+
+    /// Move the cursor to the previous node, returning [`None`] if it doesn't exist.
+    pub fn move_prev(self) -> Option<Self> {
+        self.mv(Direction::Prev)
+    }
+
+    /// Move the cursor to the next node, returning [`None`] if it doesn't exist.
+    pub fn move_next(self) -> Option<Self> {
+        self.mv(Direction::Next)
+    }
+
+    fn mv(mut self, direction: Direction) -> Option<Self> {
+        self.get_neighbor_raw(direction).map(|neighbor| Self {
+            _tree: self._tree,
+            root: self.root,
+            current: neighbor,
+        })
+    }
+
+    /// Access the previous node without moving the cursor.
+    pub fn peek_prev(&self) -> Option<(&K, &V)> {
+        self.peek(Direction::Prev)
+    }
+
+    /// Access the previous 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)> {
+        // SAFETY: `self.current` is valid by the type invariants.
+        let neighbor = unsafe {
+            match direction {
+                Direction::Prev => bindings::rb_prev(self.current),
+                Direction::Next => bindings::rb_next(self.current),
+            }
+        };
+
+        if neighbor.is_null() {
+            return None;
+        }
+
+        Some(Self::to_key_value(neighbor))
+    }
+
+    /// Access the previous node mutably without moving the cursor.
+    pub fn peek_prev_mut(&mut self) -> Option<(&K, &mut V)> {
+        self.peek_mut(Direction::Prev)
+    }
+
+    /// Access the next node mutably without moving the cursor.
+    pub fn peek_next_mut(&mut self) -> Option<(&K, &mut V)> {
+        self.peek_mut(Direction::Next)
+    }
+
+    fn peek_mut(&mut self, direction: Direction) -> Option<(&K, &mut V)> {
+        // SAFETY: `self.current` is valid by the type invariants.
+        let neighbor = unsafe {
+            match direction {
+                Direction::Prev => bindings::rb_prev(self.current),
+                Direction::Next => bindings::rb_next(self.current),
+            }
+        };
+
+        if neighbor.is_null() {
+            return None;
+        }
+
+        Some(Self::to_key_value_mut(neighbor))
+    }
+
+    fn get_neighbor_raw(&mut self, direction: Direction) -> Option<*mut bindings::rb_node> {
+        // SAFETY: `self.current` is valid by the type invariants.
+        let neighbor = unsafe {
+            match direction {
+                Direction::Prev => bindings::rb_prev(self.current),
+                Direction::Next => bindings::rb_next(self.current),
+            }
+        };
+
+        if neighbor.is_null() {
+            return None;
+        }
+
+        Some(neighbor)
+    }
+
+    // This internal method should *only* be called with a valid pointer to a node.
+    fn to_key_value(node: *mut bindings::rb_node) -> (&'a K, &'a V) {
+        // SAFETY: All links fields we create are in a `Node<K, V>`.
+        let this = unsafe { crate::container_of!(node, 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)
+    }
+
+    // This internal method should *only* be called with a valid pointer to a node.
+    fn to_key_value_mut(node: *mut bindings::rb_node) -> (&'a K, &'a mut V) {
+        // SAFETY: All links fields we create are in a `Node<K, V>`.
+        let this = unsafe { crate::container_of!(node, Node<K, V>, links) }.cast_mut();
+        // 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 { &mut (*this).value };
+        (k, v)
+    }
+
+    // This internal method should *only* be called with a valid pointer to a node *that is being removed*.
+    fn to_key_value_owned(node: *mut bindings::rb_node) -> (K, V) {
+        // SAFETY: All links fields we create are in a `Node<K, V>`.
+        let this = unsafe { crate::container_of!(node, Node<K, V>, links) }.cast_mut();
+        // SAFETY: The passed `node` is the current node or a non-null neighbor,
+        // thus `this` is valid by the type invariants.
+        let n = unsafe { Box::from_raw(this) };
+
+        (n.key, n.value)
+    }
+}
+
+/// Direction for [`RBTreeCursor`] operations.
+enum Direction {
+    /// the node immediately before, in sort order
+    Prev,
+    /// the node immediately after, in sort order
+    Next,
+}
+
 impl<'a, K, V> IntoIterator for &'a RBTree<K, V> {
     type Item = (&'a K, &'a V);
     type IntoIter = RBTreeIterator<'a, K, V>;

-- 
2.44.0.rc0.258.g7320e95886-goog

[PATCH v2 6/6] rust: rbtree: add `RBTree::entry`

[Matt Gilbride]

From: Alice Ryhl <aliceryhl@google.com>

This mirrors the entry API [1] from the Rust standard library on
`RBTree`. This API can be used to access the entry at a specific key and
make modifications depending on whether the key is vacant or occupied.
This API is useful because it can often be used to avoid traversing the
tree multiple times.

This is used by binder to look up and conditionally access or insert a
value, depending on whether it is there or not [2].

Link: https://doc.rust-lang.org/stable/std/collections/btree_map/enum.Entry.html [1]
Link: https://android-review.googlesource.com/c/kernel/common/+/2849906 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
---
 rust/kernel/rbtree.rs | 284 ++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 216 insertions(+), 68 deletions(-)

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index 6c3b10e46517..b564e4eb733c 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -298,56 +298,64 @@ pub fn try_create_and_insert(&mut self, key: K, value: V) -> Result<Option<RBTre
     ///
     /// This function always succeeds.
     pub fn insert(&mut self, node: RBTreeNode<K, V>) -> Option<RBTreeNode<K, V>> {
-        let RBTreeNode { node } = node;
-        let node = Box::into_raw(node);
-        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
-        // the node is removed or replaced.
-        let node_links = unsafe { addr_of_mut!((*node).links) };
+        match self.raw_entry(&node.node.key) {
+            RawEntry::Occupied(entry) => Some(entry.replace(node)),
+            RawEntry::Vacant(entry) => {
+                entry.insert(node);
+                None
+            }
+        }
+    }
+
+    fn raw_entry(&mut self, key: &K) -> RawEntry<'_, K, V> {
         let mut new_link: &mut *mut bindings::rb_node = &mut self.root.rb_node;
         let mut parent = core::ptr::null_mut();
-        while !new_link.is_null() {
+        while !(*new_link).is_null() {
+            let curr = *new_link;
             // SAFETY: All links fields we create are in a `Node<K, V>`.
-            let this = unsafe { crate::container_of!(*new_link, Node<K, V>, links) };
+            let node = unsafe { crate::container_of!(curr, Node<K, V>, links) };
 
-            parent = *new_link;
-
-            // SAFETY: `this` is a non-null node so it is valid by the type invariants. `node` is
-            // valid until the node is removed.
-            match unsafe { (*node).key.cmp(&(*this).key) } {
-                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
-                Ordering::Less => new_link = unsafe { &mut (*parent).rb_left },
-                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
-                Ordering::Greater => new_link = unsafe { &mut (*parent).rb_right },
+            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
+            match key.cmp(unsafe { &(*node).key }) {
+                // SAFETY: `curr` is a non-null node so it is valid by the type invariants.
+                Ordering::Less => new_link = unsafe { &mut (*curr).rb_left },
+                // SAFETY: `curr` is a non-null node so it is valid by the type invariants.
+                Ordering::Greater => new_link = unsafe { &mut (*curr).rb_right },
                 Ordering::Equal => {
-                    // INVARIANT: We are replacing an existing node with a new one, which is valid.
-                    // It remains valid because we "forgot" it with `Box::into_raw`.
-                    // SAFETY: All pointers are non-null and valid (parent, despite the name, really
-                    // is the node we're replacing).
-                    unsafe { bindings::rb_replace_node(parent, node_links, &mut self.root) };
-
-                    // INVARIANT: The node is being returned and the caller may free it, however,
-                    // it was removed from the tree. So the invariants still hold.
-                    return Some(RBTreeNode {
-                        // SAFETY: `this` was a node in the tree, so it is valid.
-                        node: unsafe { Box::from_raw(this as _) },
-                    });
+                    return RawEntry::Occupied(OccupiedEntry {
+                        rbtree: self,
+                        node_links: curr,
+                    })
                 }
             }
+            parent = curr;
         }
 
-        // INVARIANT: We are linking in a new node, which is valid. It remains valid because we
-        // "forgot" it with `Box::into_raw`.
-        // SAFETY: All pointers are non-null and valid (`*new_link` is null, but `new_link` is a
-        // mutable reference).
-        unsafe { bindings::rb_link_node(node_links, parent, new_link) };
+        RawEntry::Vacant(RawVacantEntry {
+            parent,
+            new_link,
+            rbtree: self,
+        })
+    }
 
-        // SAFETY: All pointers are valid. `node` has just been inserted into the tree.
-        unsafe { bindings::rb_insert_color(node_links, &mut self.root) };
-        None
+    /// Gets the given key's corresponding entry in the map for in-place manipulation.
+    pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
+        match self.raw_entry(&key) {
+            RawEntry::Occupied(entry) => Entry::Occupied(entry),
+            RawEntry::Vacant(entry) => Entry::Vacant(VacantEntry { raw: entry, key }),
+        }
     }
 
-    /// Returns a node with the given key, if one exists.
-    fn find(&self, key: &K) -> Option<NonNull<Node<K, V>>> {
+    /// Used for accessing the given node, if it exists.
+    pub fn find_mut(&mut self, key: &K) -> Option<OccupiedEntry<'_, K, V>> {
+        match self.raw_entry(key) {
+            RawEntry::Occupied(entry) => Some(entry),
+            RawEntry::Vacant(_entry) => None,
+        }
+    }
+
+    /// Returns a reference to the value corresponding to the key.
+    pub fn get(&self, key: &K) -> Option<&V> {
         let mut node = self.root.rb_node;
         while !node.is_null() {
             // SAFETY: All links fields we create are in a `Node<K, V>`.
@@ -358,54 +366,30 @@ fn find(&self, key: &K) -> Option<NonNull<Node<K, V>>> {
                 Ordering::Less => unsafe { (*node).rb_left },
                 // SAFETY: `node` is a non-null node so it is valid by the type invariants.
                 Ordering::Greater => unsafe { (*node).rb_right },
-                Ordering::Equal => return NonNull::new(this as _),
+                // SAFETY: `node` is a non-null node so it is valid by the type invariants.
+                Ordering::Equal => return Some(unsafe { &(*this).value }),
             }
         }
         None
     }
 
-    /// Returns a reference to the value corresponding to the key.
-    pub fn get(&self, key: &K) -> Option<&V> {
-        // SAFETY: The `find` return value is a node in the tree, so it is valid.
-        self.find(key).map(|node| unsafe { &node.as_ref().value })
-    }
-
     /// Returns a mutable reference to the value corresponding to the key.
     pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
-        // SAFETY: The `find` return value is a node in the tree, so it is valid.
-        self.find(key)
-            .map(|mut node| unsafe { &mut node.as_mut().value })
+        self.find_mut(key).map(|node| node.into_mut())
     }
 
     /// Removes the node with the given key from the tree.
     ///
     /// It returns the node that was removed if one exists, or [`None`] otherwise.
-    fn remove_node(&mut self, key: &K) -> Option<RBTreeNode<K, V>> {
-        let mut node = self.find(key)?;
-
-        // SAFETY: The `find` return value is a node in the tree, so it is valid.
-        unsafe { bindings::rb_erase(&mut node.as_mut().links, &mut self.root) };
-
-        // INVARIANT: The node is being returned and the caller may free it, however, it was
-        // removed from the tree. So the invariants still hold.
-        Some(RBTreeNode {
-            // SAFETY: The `find` return value was a node in the tree, so it is valid.
-            node: unsafe { Box::from_raw(node.as_ptr()) },
-        })
+    pub fn remove_node(&mut self, key: &K) -> Option<RBTreeNode<K, V>> {
+        self.find_mut(key).map(OccupiedEntry::remove_node)
     }
 
     /// Removes the node with the given key from the tree.
     ///
     /// It returns the value that was removed if one exists, or [`None`] otherwise.
     pub fn remove(&mut self, key: &K) -> Option<V> {
-        let node = self.remove_node(key)?;
-        let RBTreeNode { node } = node;
-        let Node {
-            links: _,
-            key: _,
-            value,
-        } = *node;
-        Some(value)
+        self.find_mut(key).map(OccupiedEntry::remove)
     }
 
     /// Returns a cursor over the tree nodes based on the given key.
@@ -1064,3 +1048,167 @@ unsafe impl<K: Send, V: Send> Send for RBTreeNode<K, V> {}
 // SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
 // fields, so we use the same Sync condition as would be used for a struct with K and V fields.
 unsafe impl<K: Sync, V: Sync> Sync for RBTreeNode<K, V> {}
+
+impl<K, V> RBTreeNode<K, V> {
+    /// "Uninitialises" a node.
+    ///
+    /// It then becomes a reservation that can be re-initialised into a different node (i.e., with
+    /// a different key and/or value).
+    ///
+    /// The existing key and value are dropped in-place as part of this operation, that is, memory
+    /// may be freed (but only for the key/value; memory for the node itself is kept for reuse).
+    pub fn into_reservation(self) -> RBTreeNodeReservation<K, V> {
+        let raw = Box::into_raw(self.node);
+        let mut ret = RBTreeNodeReservation {
+            // SAFETY: The pointer came from a valid `Node`, which has the same layout as
+            // `MaybeUninit<Node>`.
+            node: unsafe { Box::from_raw(raw as _) },
+        };
+        // SAFETY: Although the type is `MaybeUninit<Node>`, we know it has been initialised
+        // because it came from a `Node`. So it is safe to drop it.
+        unsafe { core::ptr::drop_in_place(ret.node.as_mut_ptr()) };
+        ret
+    }
+}
+
+/// A view into a single entry in a map, which may either be vacant or occupied.
+///
+/// This enum is constructed from the [`entry`] method on [`RBTree`].
+///
+/// [`entry`]: fn@RBTree::entry
+pub enum Entry<'a, K, V> {
+    /// This [`RBTree`] does not have a node with this key.
+    Vacant(VacantEntry<'a, K, V>),
+    /// This [`RBTree`] already has a node with this key.
+    Occupied(OccupiedEntry<'a, K, V>),
+}
+
+/// Like [`Entry`], except that it doesn't have ownership of the key.
+enum RawEntry<'a, K, V> {
+    Vacant(RawVacantEntry<'a, K, V>),
+    Occupied(OccupiedEntry<'a, K, V>),
+}
+
+/// A view into a vacant entry in a [`RBTree`]. It is part of the [`Entry`] enum.
+pub struct VacantEntry<'a, K, V> {
+    key: K,
+    raw: RawVacantEntry<'a, K, V>,
+}
+
+/// Like [`VacantEntry`], but doesn't hold on to the key.
+struct RawVacantEntry<'a, K, V> {
+    rbtree: &'a mut RBTree<K, V>,
+    /// The node that will become the parent of the new node if we insert one.
+    ///
+    /// This pointer may be null if the new node becomes the root.
+    parent: *mut bindings::rb_node,
+    /// This points to the left-child or right-child field of `parent`. This controls whether the
+    /// new node will become the left or right child of `parent`.
+    ///
+    /// If `parent` is null, then this points at `rbtree.root`.
+    new_link: *mut *mut bindings::rb_node,
+}
+
+impl<'a, K, V> RawVacantEntry<'a, K, V> {
+    /// Inserts the given node into the [`RBTree`] at this entry.
+    ///
+    /// The `node` must have a key such that inserting it here does not break the ordering of this
+    /// [`RBTree`].
+    fn insert(self, node: RBTreeNode<K, V>) -> &'a mut V {
+        let node = Box::into_raw(node.node);
+
+        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
+        // the node is removed or replaced.
+        let node_links = unsafe { addr_of_mut!((*node).links) };
+
+        // INVARIANT: We are linking in a new node, which is valid. It remains valid because we
+        // "forgot" it with `Box::into_raw`.
+        // SAFETY: All pointers are null or valid in an appropriate way.
+        unsafe { bindings::rb_link_node(node_links, self.parent, self.new_link) };
+
+        // SAFETY: All pointers are valid. `node` has just been inserted into the tree.
+        unsafe { bindings::rb_insert_color(node_links, &mut self.rbtree.root) };
+
+        // SAFETY: The node is valid until we remove it from the tree.
+        unsafe { &mut (*node).value }
+    }
+}
+
+impl<'a, K, V> VacantEntry<'a, K, V> {
+    /// Inserts the given node into the [`RBTree`] at this entry.
+    pub fn insert(self, value: V, reservation: RBTreeNodeReservation<K, V>) -> &'a mut V {
+        self.raw.insert(reservation.into_node(self.key, value))
+    }
+}
+
+/// A view into an occupied entry in a [`RBTree`]. It is part of the [`Entry`] enum.
+pub struct OccupiedEntry<'a, K, V> {
+    rbtree: &'a mut RBTree<K, V>,
+    /// The node that this entry corresponds to. Non null.
+    node_links: *mut bindings::rb_node,
+}
+
+impl<'a, K, V> OccupiedEntry<'a, K, V> {
+    fn node_ptr(&self) -> *mut Node<K, V> {
+        // SAFETY: All links fields we create are in a `Node<K, V>`.
+        unsafe { crate::container_of!(self.node_links, Node<K, V>, links) }.cast_mut()
+    }
+
+    /// Gets a reference to the value in the entry.
+    pub fn get(&self) -> &V {
+        unsafe { &(*self.node_ptr()).value }
+    }
+
+    /// Gets a mutable reference to the value in the entry.
+    pub fn get_mut(&mut self) -> &mut V {
+        unsafe { &mut (*self.node_ptr()).value }
+    }
+
+    /// Converts the entry into a mutable reference to its value.
+    ///
+    /// If you need multiple references to the `OccupiedEntry`, see [`self#get_mut`].
+    pub fn into_mut(self) -> &'a mut V {
+        unsafe { &mut (*self.node_ptr()).value }
+    }
+
+    /// Remove this entry from the [`RBTree`].
+    pub fn remove_node(self) -> RBTreeNode<K, V> {
+        // SAFETY: The node is a node in the tree, so it is valid.
+        unsafe { bindings::rb_erase(self.node_links, &mut self.rbtree.root) };
+
+        // INVARIANT: The node is being returned and the caller may free it, however, it was
+        // removed from the tree. So the invariants still hold.
+        RBTreeNode {
+            // SAFETY: The node was a node in the tree, but we removed it, so we can convert it
+            // back into a box.
+            node: unsafe { Box::from_raw(self.node_ptr()) },
+        }
+    }
+
+    /// Takes the value of the entry out of the map, and returns it.
+    pub fn remove(self) -> V {
+        self.remove_node().node.value
+    }
+
+    /// Swap the current node for the provided node.
+    ///
+    /// The key of both nodes must be equal.
+    fn replace(self, node: RBTreeNode<K, V>) -> RBTreeNode<K, V> {
+        let node = Box::into_raw(node.node);
+
+        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
+        // the node is removed or replaced.
+        let new_node_links = unsafe { addr_of_mut!((*node).links) };
+
+        // SAFETY: This updates the pointers so that `new_node_links` is in the tree where
+        // `self.node_links` used to be.
+        unsafe {
+            bindings::rb_replace_node(self.node_links, new_node_links, &mut self.rbtree.root)
+        };
+
+        // SAFETY: Now that we removed this entry from the tree, we can convert the node to a box.
+        let old_node = unsafe { Box::from_raw(self.node_ptr()) };
+
+        RBTreeNode { node: old_node }
+    }
+}

-- 
2.44.0.rc0.258.g7320e95886-goog

Re: [PATCH v2 1/6] rust: add `container_of!` macro

[Miguel Ojeda]

On Mon, Feb 19, 2024 at 12:48 PM Matt Gilbride <mattgilbride@google.com> wrote:
>
> From: Wedson Almeida Filho <wedsonaf@gmail.com>
>
> This macro is used to obtain a pointer to an entire struct
> when given a pointer to a field in that struct.
>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Tested-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Matt Gilbride <mattgilbride@google.com>

Applied to `rust-next` -- thanks everyone!

I took #1 of this series for the time being so that we have
`container_of!` in already since others were waiting for it.

Cheers,
Miguel

Re: [PATCH v2 2/6] rust: rbtree: add red-black tree implementation backed by the C version

[Benno Lossin]

On 2/19/24 12:48, Matt Gilbride wrote:
> +impl<K, V> RBTree<K, V>
> +where
> +    K: Ord,
> +{
> +    /// Tries to insert a new value into the tree.
> +    ///
> +    /// It overwrites a node if one already exists with the same key and returns it (containing the
> +    /// key/value pair). Returns [`None`] if a node with the same key didn't already exist.
> +    ///
> +    /// Returns an error if it cannot allocate memory for the new node.
> +    pub fn try_create_and_insert(&mut self, key: K, value: V) -> Result<Option<RBTreeNode<K, V>>> {
> +        Ok(self.insert(Self::try_allocate_node(key, value)?))
> +    }
> +
> +    /// Inserts a new node into the tree.
> +    ///
> +    /// It overwrites a node if one already exists with the same key and returns it (containing the
> +    /// key/value pair). Returns [`None`] if a node with the same key didn't already exist.
> +    ///
> +    /// This function always succeeds.
> +    pub fn insert(&mut self, node: RBTreeNode<K, V>) -> Option<RBTreeNode<K, V>> {
> +        let RBTreeNode { node } = node;

You can do this pattern deconstruction directly in the function
signature.

> +        let node = Box::into_raw(node);
> +        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
> +        // the node is removed or replaced.
> +        let node_links = unsafe { addr_of_mut!((*node).links) };
> +        let mut new_link: &mut *mut bindings::rb_node = &mut self.root.rb_node;

Is the only reason for this being a double pointer that `rb_link_node`
requires that as its last argument? If yes, then I would just add a
`&mut` there and give this the simpler type.

Also a more fitting name IMO would be `cur_node` or similar.

> +        let mut parent = core::ptr::null_mut();

I would suggest naming this `prev_node` or similar.

> +        while !new_link.is_null() {
> +            // SAFETY: All links fields we create are in a `Node<K, V>`.

Suggestion:
     // 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 { crate::container_of!(*new_link, Node<K, V>, links) };

The `container_of` macro is used 3 times, I think it's nicer to import
it.

> +
> +            parent = *new_link;
> +
> +            // SAFETY: `this` is a non-null node so it is valid by the type invariants. `node` is
> +            // valid until the node is removed.
> +            match unsafe { (*node).key.cmp(&(*this).key) } {
> +                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
> +                Ordering::Less => new_link = unsafe { &mut (*parent).rb_left },

I would use `*new_link` instead of `parent` here.

> +                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
> +                Ordering::Greater => new_link = unsafe { &mut (*parent).rb_right },

Ditto.

> +                Ordering::Equal => {
> +                    // INVARIANT: We are replacing an existing node with a new one, which is valid.
> +                    // It remains valid because we "forgot" it with `Box::into_raw`.
> +                    // SAFETY: All pointers are non-null and valid (parent, despite the name, really
> +                    // is the node we're replacing).
> +                    unsafe { bindings::rb_replace_node(parent, node_links, &mut self.root) };

If you use `*new_link` instead of `parent` (and perform the rename) then
you don't need the note in the parenthesis in the safety comment.

> +
> +                    // INVARIANT: The node is being returned and the caller may free it, however,
> +                    // it was removed from the tree. So the invariants still hold.
> +                    return Some(RBTreeNode {
> +                        // SAFETY: `this` was a node in the tree, so it is valid.
> +                        node: unsafe { Box::from_raw(this as _) },

Why do you need this cast? Can it be replaced by `.cast()` or
`.cast_mut()`?

> +                    });
> +                }
> +            }
> +        }
> +
> +        // INVARIANT: We are linking in a new node, which is valid. It remains valid because we
> +        // "forgot" it with `Box::into_raw`.
> +        // SAFETY: All pointers are non-null and valid (`*new_link` is null, but `new_link` is a
> +        // mutable reference).
> +        unsafe { bindings::rb_link_node(node_links, parent, new_link) };
> +
> +        // SAFETY: All pointers are valid. `node` has just been inserted into the tree.
> +        unsafe { bindings::rb_insert_color(node_links, &mut self.root) };
> +        None
> +    }
> +
> +    /// Returns a node with the given key, if one exists.
> +    fn find(&self, key: &K) -> Option<NonNull<Node<K, V>>> {
> +        let mut node = self.root.rb_node;
> +        while !node.is_null() {
> +            // SAFETY: All links fields we create are in a `Node<K, V>`.

See above suggestion.

> +            let this = unsafe { crate::container_of!(node, Node<K, V>, links) };
> +            // SAFETY: `this` is a non-null node so it is valid by the type invariants.
> +            node = match key.cmp(unsafe { &(*this).key }) {
> +                // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +                Ordering::Less => unsafe { (*node).rb_left },
> +                // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +                Ordering::Greater => unsafe { (*node).rb_right },
> +                Ordering::Equal => return NonNull::new(this as _),

Why do you need this cast? Can it be replaced by `.cast()` or
`.cast_mut()`?

> +            }
> +        }

If you modify this function to return the parent node if `key` were in
the tree, then you could use this in `insert` instead of having to write
two loops.

> +        None
> +    }
> +
> +    /// Returns a reference to the value corresponding to the key.
> +    pub fn get(&self, key: &K) -> Option<&V> {
> +        // SAFETY: The `find` return value is a node in the tree, so it is valid.
> +        self.find(key).map(|node| unsafe { &node.as_ref().value })
> +    }
> +
> +    /// Returns a mutable reference to the value corresponding to the key.
> +    pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
> +        // SAFETY: The `find` return value is a node in the tree, so it is valid.
> +        self.find(key)
> +            .map(|mut node| unsafe { &mut node.as_mut().value })
> +    }
> +
> +    /// Removes the node with the given key from the tree.
> +    ///
> +    /// It returns the node that was removed if one exists, or [`None`] otherwise.
> +    fn remove_node(&mut self, key: &K) -> Option<RBTreeNode<K, V>> {
> +        let mut node = self.find(key)?;
> +
> +        // SAFETY: The `find` return value is a node in the tree, so it is valid.
> +        unsafe { bindings::rb_erase(&mut node.as_mut().links, &mut self.root) };
> +
> +        // INVARIANT: The node is being returned and the caller may free it, however, it was
> +        // removed from the tree. So the invariants still hold.
> +        Some(RBTreeNode {
> +            // SAFETY: The `find` return value was a node in the tree, so it is valid.
> +            node: unsafe { Box::from_raw(node.as_ptr()) },
> +        })
> +    }
> +
> +    /// Removes the node with the given key from the tree.
> +    ///
> +    /// It returns the value that was removed if one exists, or [`None`] otherwise.
> +    pub fn remove(&mut self, key: &K) -> Option<V> {
> +        let node = self.remove_node(key)?;
> +        let RBTreeNode { node } = node;
> +        let Node {
> +            links: _,
> +            key: _,
> +            value,
> +        } = *node;
> +        Some(value)

This could be a one-liner:
     self.remove_node(key).map(|node| node.node.value)

> +    }
> +}
> +
> +impl<K, V> Default for RBTree<K, V> {
> +    fn default() -> Self {
> +        Self::new()
> +    }
> +}
> +
> +impl<K, V> Drop for RBTree<K, V> {
> +    fn drop(&mut self) {
> +        // SAFETY: `root` is valid as it's embedded in `self` and we have a valid `self`.
> +        let mut next = unsafe { bindings::rb_first_postorder(&self.root) };
> +
> +        // INVARIANT: The loop invariant is that all tree nodes from `next` in postorder are valid.
> +        while !next.is_null() {
> +            // SAFETY: All links fields we create are in a `Node<K, V>`.
> +            let this = unsafe { crate::container_of!(next, Node<K, V>, links) };
> +
> +            // Find out what the next node is before disposing of the current one.
> +            // SAFETY: `next` and all nodes in postorder are still valid.
> +            next = unsafe { bindings::rb_next_postorder(next) };
> +
> +            // INVARIANT: This is the destructor, so we break the type invariant during clean-up,
> +            // but it is not observable. The loop invariant is still maintained.
> +            // SAFETY: `this` is valid per the loop invariant.
> +            unsafe { drop(Box::from_raw(this as *mut Node<K, V>)) };

Use `.cast_mut()` instead of `as ...`.

> +        }
> +    }
> +}
> +
> +/// 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
> +/// can be obtained by directly allocating it ([`RBTree::try_reserve_node`]).
> +pub struct RBTreeNodeReservation<K, V> {
> +    node: Box<MaybeUninit<Node<K, V>>>,
> +}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Send condition as would be used for a struct with K and V fields.
> +unsafe impl<K: Send, V: Send> Send for RBTreeNodeReservation<K, V> {}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
> +unsafe impl<K: Sync, V: Sync> Sync for RBTreeNodeReservation<K, V> {}

The two safety comments are copy-pasted. `RBTreeNodeReservation` does not
allow any kind of access to its values.

> +
> +impl<K, V> RBTreeNodeReservation<K, V> {
> +    /// Initialises a node reservation.
> +    ///
> +    /// It then becomes an [`RBTreeNode`] that can be inserted into a tree.
> +    pub fn into_node(mut self, key: K, value: V) -> RBTreeNode<K, V> {
> +        let node_ptr = self.node.as_mut_ptr();
> +        // SAFETY: `node_ptr` is valid, and so are its fields.
> +        unsafe { addr_of_mut!((*node_ptr).links).write(bindings::rb_node::default()) };
> +        // SAFETY: `node_ptr` is valid, and so are its fields.
> +        unsafe { addr_of_mut!((*node_ptr).key).write(key) };
> +        // SAFETY: `node_ptr` is valid, and so are its fields.
> +        unsafe { addr_of_mut!((*node_ptr).value).write(value) };
> +        let raw = Box::into_raw(self.node);
> +        RBTreeNode {
> +            // SAFETY: The pointer came from a `MaybeUninit<Node>` whose fields have all been
> +            // initialised. Additionally, it has the same layout as `Node`.
> +            node: unsafe { Box::from_raw(raw as _) },
> +        }

Instead of doing `into_raw` and `from_raw`, I would use
`Box::assume_init` (it is unstable via `new_uninit`).

> +    }
> +}
> +
> +/// A red-black tree node.
> +///
> +/// The node is fully initialised (with key and value) and can be inserted into a tree without any
> +/// extra allocations or failure paths.
> +pub struct RBTreeNode<K, V> {
> +    node: Box<Node<K, V>>,
> +}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Send condition as would be used for a struct with K and V fields.
> +unsafe impl<K: Send, V: Send> Send for RBTreeNode<K, V> {}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
> +unsafe impl<K: Sync, V: Sync> Sync for RBTreeNode<K, V> {}

The two safety comments are copy-pasted. `RBTreeNode` does not
allow any kind of access to its values.

-- 
Cheers,
Benno

> 
> --
> 2.44.0.rc0.258.g7320e95886-goog
> 

Re: [PATCH v2 3/6] rust: rbtree: add `RBTreeIterator`

[Benno Lossin]

On 2/19/24 12:48, Matt Gilbride wrote:
> @@ -350,6 +393,52 @@ fn drop(&mut self) {
>       }
>   }
> 
> +impl<'a, K, V> IntoIterator for &'a RBTree<K, V> {
> +    type Item = (&'a K, &'a V);
> +    type IntoIter = RBTreeIterator<'a, K, V>;
> +
> +    fn into_iter(self) -> Self::IntoIter {
> +        self.iter()
> +    }
> +}
> +
> +/// An iterator over the nodes of a [`RBTree`].
> +///
> +/// Instances are created by calling [`RBTree::iter`].

There probably should be some invariants here, like
- `self.next` is a valid pointer
- `self.next` points to a node stored inside of a valid `RBtree`

> +pub struct RBTreeIterator<'a, K, V> {
> +    _tree: PhantomData<&'a RBTree<K, V>>,
> +    next: *mut bindings::rb_node,
> +}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Send condition as would be used for a struct with K and V fields.
> +unsafe impl<'a, K: Send, V: Send> Send for RBTreeIterator<'a, K, V> {}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
> +unsafe impl<'a, K: Sync, V: Sync> Sync for RBTreeIterator<'a, K, V> {}

These comments should also be updated.

> +
> +impl<'a, K, V> Iterator for RBTreeIterator<'a, K, V> {
> +    type Item = (&'a K, &'a V);
> +
> +    fn next(&mut self) -> Option<Self::Item> {
> +        if self.next.is_null() {
> +            return None;
> +        }
> +
> +        // SAFETY: All links fields we create are in a `Node<K, V>`.

See my suggestion on patch 2.

> +        let cur = unsafe { crate::container_of!(self.next, Node<K, V>, links) };
> +
> +        // SAFETY: The reference to the tree used to create the iterator outlives the iterator, so
> +        // the tree cannot change. By the tree invariant, all nodes are valid.

Why does the pointer come from a tree to begin with? (ie you need the
invariants I stated above)

-- 
Cheers,
Benno

> +        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) })
> +    }
> +}
> +
>   /// 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.rc0.258.g7320e95886-goog
> 

Re: [PATCH v2 4/6] rust: rbtree: add `RBTreeIteratorMut`

[Benno Lossin]

On 2/19/24 12:48, Matt Gilbride wrote:
> 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>

The comments from patch 3 also apply here.

-- 
Cheers,
Benno

Re: [PATCH v2 5/6] rust: rbtree: add `RBTreeCursor`

[Benno Lossin]

On 2/19/24 12:48, Matt Gilbride wrote:
> Add a cursor interface to `RBTree`, supporting the following use cases:
> - Inspect the current node pointed to by the cursor, inspect/move to
>    it's neighbors in sort order (bidirectionally).
> - Mutate the tree itself by removing the current node pointed to by the
>    cursor, or one of its neighbors.
> 
> Add functions to obtain a cursor to the tree by key:
> - The node with the smallest key
> - The node with the largest key
> - The node matching the given key, or the one with the next larger key
> 
> The cursor abstraction is needed by the binder driver to efficiently
> search for nodes and (conditionally) modify them, as well as their
> neighbors [1].
> 
> Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-6-08ba9197f637@google.com/ [1]
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Tested-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Matt Gilbride <mattgilbride@google.com>
> ---
>   rust/kernel/rbtree.rs | 512 ++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 512 insertions(+)
> 
> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index ccf74e0dc3ec..6c3b10e46517 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -245,6 +245,36 @@ pub fn values(&self) -> impl Iterator<Item = &'_ V> {
>       pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
>           self.iter_mut().map(|(_, v)| v)
>       }
> +
> +    /// Returns a cursor over the tree nodes, starting with the smallest key.
> +    pub fn cursor_front(&mut self) -> Option<RBTreeCursor<'_, K, V>> {
> +        let root = addr_of_mut!(self.root);
> +        // SAFETY: `self.root` is always a valid root node
> +        let current = unsafe { bindings::rb_first(root) };
> +        if current.is_null() {
> +            return None;
> +        }
> +        Some(RBTreeCursor {

Missing INVARIANT comment.

> +            _tree: PhantomData,
> +            root,
> +            current,
> +        })
> +    }
> +
> +    /// Returns a cursor over the tree nodes, starting with the largest key.
> +    pub fn cursor_back(&mut self) -> Option<RBTreeCursor<'_, K, V>> {
> +        let root = addr_of_mut!(self.root);
> +        // SAFETY: `self.root` is always a valid root node
> +        let current = unsafe { bindings::rb_last(root) };
> +        if current.is_null() {
> +            return None;
> +        }
> +        Some(RBTreeCursor {

Ditto.

> +            _tree: PhantomData,
> +            root,
> +            current,
> +        })
> +    }
>   }
> 
>   impl<K, V> RBTree<K, V>
> @@ -377,6 +407,59 @@ pub fn remove(&mut self, key: &K) -> Option<V> {
>           } = *node;
>           Some(value)
>       }
> +
> +    /// 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(&mut self, key: &K) -> Option<RBTreeCursor<'_, K, V>>
> +    where
> +        K: Ord,
> +    {
> +        let mut node = self.root.rb_node;
> +        let mut best_match: Option<NonNull<Node<K, V>>> = None;
> +        while !node.is_null() {
> +            // SAFETY: All links fields we create are in a `Node<K, V>`.
> +            let this = unsafe { crate::container_of!(node, Node<K, V>, links) }.cast_mut();
> +            // SAFETY: `this` is a non-null node so it is valid by the type invariants.
> +            let this_key = unsafe { &(*this).key };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let left_child = unsafe { (*node).rb_left };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let right_child = unsafe { (*node).rb_right };
> +            if key == this_key {
> +                return Some(RBTreeCursor {

Ditto.

> +                    _tree: PhantomData,
> +                    root: addr_of_mut!(self.root),
> +                    current: node,
> +                });
> +            } else {
> +                node = if key > this_key {
> +                    right_child
> +                } else {
> +                    let is_better_match = match best_match {
> +                        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
> +                        }
> +                    };
> +                    if is_better_match {
> +                        best_match = NonNull::new(this);
> +                    }
> +                    left_child
> +                }
> +            };
> +        }
> +        best_match.map(|best| RBTreeCursor {

Ditto.

> +            _tree: PhantomData,
> +            root: addr_of_mut!(self.root),
> +            // SAFETY: `best` is a non-null node so it is valid by the type invariants.
> +            current: unsafe { addr_of_mut!((*best.as_ptr()).links) },
> +        })
> +    }
>   }
> 
>   impl<K, V> Default for RBTree<K, V> {
> @@ -407,6 +490,435 @@ fn drop(&mut self) {
>       }
>   }
> 
> +/// A bidirectional cursor over the tree nodes, sorted by key.
> +///
> +/// # Invariants
> +///
> +/// In instance of `RBTreeCursor` is only acquired from [`RBTree`].
> +/// A reference to the tree used to create the cursor outlives the cursor, so
> +/// the tree cannot change. By the tree invariant, all nodes are valid.

Make the invariant mention the fields directly:
- `root` and `current` are valid pointers
- `root` points to the `root` node of an [`RBTree`]
- `current` points to a node that is in the same [`RBTree`] that `root` is pointing to

> +///
> +/// # Examples

[...]

> +pub struct RBTreeCursor<'a, K, V> {
> +    _tree: PhantomData<&'a RBTree<K, V>>,
> +    root: *mut bindings::rb_root,
> +    current: *mut bindings::rb_node,
> +}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Send condition as would be used for a struct with K and V fields.
> +unsafe impl<'a, K: Send, V: Send> Send for RBTreeCursor<'a, K, V> {}
> +
> +// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
> +// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
> +unsafe impl<'a, K: Sync, V: Sync> Sync for RBTreeCursor<'a, K, V> {}
> +
> +impl<'a, K, V> RBTreeCursor<'a, K, V> {
> +    /// The current node
> +    pub fn current(&self) -> (&K, &V) {
> +        Self::to_key_value(self.current)
> +    }
> +
> +    /// The current node, with a mutable value
> +    pub fn current_mut(&mut self) -> (&K, &mut V) {
> +        Self::to_key_value_mut(self.current)
> +    }
> +
> +    /// Remove the current node from the tree.
> +    ///
> +    /// Returns a cursor to the next node, if it exists,
> +    /// else the previous node. Returns [`None`] if the tree
> +    /// becomes empty.
> +    pub fn remove_current(mut self) -> Option<Self> {
> +        let prev = self.get_neighbor_raw(Direction::Prev);
> +        let next = self.get_neighbor_raw(Direction::Next);
> +        // SAFETY: All links fields we create are in a `Node<K, V>`.

This safety comment should be updated like the ones in the earlier
patches.

> +        let this = unsafe { crate::container_of!(self.current, Node<K, V>, links) }.cast_mut();
> +        // SAFETY: The reference to the tree used to create the cursor outlives the cursor, so
> +        // the tree cannot change. By the tree invariant, all nodes are valid.
> +        unsafe { bindings::rb_erase(&mut (*this).links, self.root) };
> +
> +        let current = match (prev, next) {
> +            (_, Some(next)) => next,
> +            (Some(prev), None) => prev,
> +            (None, None) => {
> +                return None;
> +            }
> +        };
> +
> +        Some(Self {

Missing INVARIANT comment.

> +            current,
> +            _tree: self._tree,
> +            root: self.root,
> +        })
> +    }
> +
> +    /// Remove the previous node, returning it if it exists.
> +    pub fn remove_prev(&mut self) -> Option<(K, V)> {
> +        self.remove_neighbor(Direction::Prev)
> +    }
> +
> +    /// Remove the next node, returning it if it exists.
> +    pub fn remove_next(&mut self) -> Option<(K, V)> {
> +        self.remove_neighbor(Direction::Next)
> +    }
> +
> +    fn remove_neighbor(&mut self, direction: Direction) -> Option<(K, V)> {
> +        if let Some(neighbor) = self.get_neighbor_raw(direction) {
> +            // SAFETY: All links fields we create are in a `Node<K, V>`.
> +            let this = unsafe { crate::container_of!(neighbor, Node<K, V>, links) }.cast_mut();
> +            // SAFETY: The reference to the tree used to create the cursor outlives the cursor, so
> +            // the tree cannot change. By the tree invariant, all nodes are valid.
> +            unsafe { bindings::rb_erase(&mut (*this).links, self.root) };
> +            return Some(Self::to_key_value_owned(neighbor));
> +        }
> +        None
> +    }
> +
> +    /// Move the cursor to the previous node, returning [`None`] if it doesn't exist.
> +    pub fn move_prev(self) -> Option<Self> {
> +        self.mv(Direction::Prev)
> +    }
> +
> +    /// Move the cursor to the next node, returning [`None`] if it doesn't exist.
> +    pub fn move_next(self) -> Option<Self> {
> +        self.mv(Direction::Next)
> +    }
> +
> +    fn mv(mut self, direction: Direction) -> Option<Self> {
> +        self.get_neighbor_raw(direction).map(|neighbor| Self {

Ditto.

> +            _tree: self._tree,
> +            root: self.root,
> +            current: neighbor,
> +        })
> +    }

[...]

> +    fn peek_mut(&mut self, direction: Direction) -> Option<(&K, &mut V)> {
> +        // SAFETY: `self.current` is valid by the type invariants.
> +        let neighbor = unsafe {
> +            match direction {
> +                Direction::Prev => bindings::rb_prev(self.current),
> +                Direction::Next => bindings::rb_next(self.current),
> +            }
> +        };
> +
> +        if neighbor.is_null() {
> +            return None;
> +        }

Why not use `get_neighbor_raw` here?

> +
> +        Some(Self::to_key_value_mut(neighbor))
> +    }
> +
> +    fn get_neighbor_raw(&mut self, direction: Direction) -> Option<*mut bindings::rb_node> {
> +        // SAFETY: `self.current` is valid by the type invariants.
> +        let neighbor = unsafe {
> +            match direction {
> +                Direction::Prev => bindings::rb_prev(self.current),
> +                Direction::Next => bindings::rb_next(self.current),
> +            }
> +        };
> +
> +        if neighbor.is_null() {
> +            return None;
> +        }
> +
> +        Some(neighbor)
> +    }
> +
> +    // This internal method should *only* be called with a valid pointer to a node.
> +    fn to_key_value(node: *mut bindings::rb_node) -> (&'a K, &'a V) {
> +        // SAFETY: All links fields we create are in a `Node<K, V>`.
> +        let this = unsafe { crate::container_of!(node, 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)
> +    }
> +
> +    // This internal method should *only* be called with a valid pointer to a node.
> +    fn to_key_value_mut(node: *mut bindings::rb_node) -> (&'a K, &'a mut V) {
> +        // SAFETY: All links fields we create are in a `Node<K, V>`.
> +        let this = unsafe { crate::container_of!(node, Node<K, V>, links) }.cast_mut();
> +        // 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 { &mut (*this).value };
> +        (k, v)
> +    }
> +
> +    // This internal method should *only* be called with a valid pointer to a node *that is being removed*.
> +    fn to_key_value_owned(node: *mut bindings::rb_node) -> (K, V) {
> +        // SAFETY: All links fields we create are in a `Node<K, V>`.
> +        let this = unsafe { crate::container_of!(node, Node<K, V>, links) }.cast_mut();
> +        // SAFETY: The passed `node` is the current node or a non-null neighbor,
> +        // thus `this` is valid by the type invariants.
> +        let n = unsafe { Box::from_raw(this) };
> +
> +        (n.key, n.value)
> +    }

These internal methods should be `unsafe` and have a Safety section.

-- 
Cheers,
Benno

> +}
> +
> +/// Direction for [`RBTreeCursor`] operations.
> +enum Direction {
> +    /// the node immediately before, in sort order
> +    Prev,
> +    /// the node immediately after, in sort order
> +    Next,
> +}
> +
>   impl<'a, K, V> IntoIterator for &'a RBTree<K, V> {
>       type Item = (&'a K, &'a V);
>       type IntoIter = RBTreeIterator<'a, K, V>;
> 
> --
> 2.44.0.rc0.258.g7320e95886-goog
> 

Re: [PATCH v2 6/6] rust: rbtree: add `RBTree::entry`

[Benno Lossin]

On 2/19/24 12:48, Matt Gilbride wrote:
> From: Alice Ryhl <aliceryhl@google.com>
> 
> This mirrors the entry API [1] from the Rust standard library on
> `RBTree`. This API can be used to access the entry at a specific key and
> make modifications depending on whether the key is vacant or occupied.
> This API is useful because it can often be used to avoid traversing the
> tree multiple times.
> 
> This is used by binder to look up and conditionally access or insert a
> value, depending on whether it is there or not [2].
> 
> Link: https://doc.rust-lang.org/stable/std/collections/btree_map/enum.Entry.html [1]
> Link: https://android-review.googlesource.com/c/kernel/common/+/2849906 [2]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Tested-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Matt Gilbride <mattgilbride@google.com>
> ---
>   rust/kernel/rbtree.rs | 284 ++++++++++++++++++++++++++++++++++++++------------
>   1 file changed, 216 insertions(+), 68 deletions(-)
> 
> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index 6c3b10e46517..b564e4eb733c 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -298,56 +298,64 @@ pub fn try_create_and_insert(&mut self, key: K, value: V) -> Result<Option<RBTre
>       ///
>       /// This function always succeeds.
>       pub fn insert(&mut self, node: RBTreeNode<K, V>) -> Option<RBTreeNode<K, V>> {
> -        let RBTreeNode { node } = node;
> -        let node = Box::into_raw(node);
> -        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
> -        // the node is removed or replaced.
> -        let node_links = unsafe { addr_of_mut!((*node).links) };
> +        match self.raw_entry(&node.node.key) {
> +            RawEntry::Occupied(entry) => Some(entry.replace(node)),
> +            RawEntry::Vacant(entry) => {
> +                entry.insert(node);
> +                None
> +            }
> +        }
> +    }
> +
> +    fn raw_entry(&mut self, key: &K) -> RawEntry<'_, K, V> {
>           let mut new_link: &mut *mut bindings::rb_node = &mut self.root.rb_node;
>           let mut parent = core::ptr::null_mut();
> -        while !new_link.is_null() {
> +        while !(*new_link).is_null() {
> +            let curr = *new_link;
>               // SAFETY: All links fields we create are in a `Node<K, V>`.
> -            let this = unsafe { crate::container_of!(*new_link, Node<K, V>, links) };
> +            let node = unsafe { crate::container_of!(curr, Node<K, V>, links) };
> 
> -            parent = *new_link;
> -
> -            // SAFETY: `this` is a non-null node so it is valid by the type invariants. `node` is
> -            // valid until the node is removed.
> -            match unsafe { (*node).key.cmp(&(*this).key) } {
> -                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
> -                Ordering::Less => new_link = unsafe { &mut (*parent).rb_left },
> -                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
> -                Ordering::Greater => new_link = unsafe { &mut (*parent).rb_right },
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            match key.cmp(unsafe { &(*node).key }) {
> +                // SAFETY: `curr` is a non-null node so it is valid by the type invariants.
> +                Ordering::Less => new_link = unsafe { &mut (*curr).rb_left },
> +                // SAFETY: `curr` is a non-null node so it is valid by the type invariants.
> +                Ordering::Greater => new_link = unsafe { &mut (*curr).rb_right },
>                   Ordering::Equal => {
> -                    // INVARIANT: We are replacing an existing node with a new one, which is valid.
> -                    // It remains valid because we "forgot" it with `Box::into_raw`.
> -                    // SAFETY: All pointers are non-null and valid (parent, despite the name, really
> -                    // is the node we're replacing).
> -                    unsafe { bindings::rb_replace_node(parent, node_links, &mut self.root) };
> -
> -                    // INVARIANT: The node is being returned and the caller may free it, however,
> -                    // it was removed from the tree. So the invariants still hold.
> -                    return Some(RBTreeNode {
> -                        // SAFETY: `this` was a node in the tree, so it is valid.
> -                        node: unsafe { Box::from_raw(this as _) },
> -                    });
> +                    return RawEntry::Occupied(OccupiedEntry {
> +                        rbtree: self,
> +                        node_links: curr,
> +                    })
>                   }
>               }
> +            parent = curr;
>           }
> 
> -        // INVARIANT: We are linking in a new node, which is valid. It remains valid because we
> -        // "forgot" it with `Box::into_raw`.
> -        // SAFETY: All pointers are non-null and valid (`*new_link` is null, but `new_link` is a
> -        // mutable reference).
> -        unsafe { bindings::rb_link_node(node_links, parent, new_link) };
> +        RawEntry::Vacant(RawVacantEntry {
> +            parent,
> +            new_link,
> +            rbtree: self,
> +        })
> +    }
> 
> -        // SAFETY: All pointers are valid. `node` has just been inserted into the tree.
> -        unsafe { bindings::rb_insert_color(node_links, &mut self.root) };
> -        None
> +    /// Gets the given key's corresponding entry in the map for in-place manipulation.
> +    pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
> +        match self.raw_entry(&key) {
> +            RawEntry::Occupied(entry) => Entry::Occupied(entry),
> +            RawEntry::Vacant(entry) => Entry::Vacant(VacantEntry { raw: entry, key }),
> +        }
>       }
> 
> -    /// Returns a node with the given key, if one exists.
> -    fn find(&self, key: &K) -> Option<NonNull<Node<K, V>>> {
> +    /// Used for accessing the given node, if it exists.
> +    pub fn find_mut(&mut self, key: &K) -> Option<OccupiedEntry<'_, K, V>> {
> +        match self.raw_entry(key) {
> +            RawEntry::Occupied(entry) => Some(entry),
> +            RawEntry::Vacant(_entry) => None,
> +        }
> +    }
> +
> +    /// Returns a reference to the value corresponding to the key.
> +    pub fn get(&self, key: &K) -> Option<&V> {
>           let mut node = self.root.rb_node;
>           while !node.is_null() {
>               // SAFETY: All links fields we create are in a `Node<K, V>`.
> @@ -358,54 +366,30 @@ fn find(&self, key: &K) -> Option<NonNull<Node<K, V>>> {
>                   Ordering::Less => unsafe { (*node).rb_left },
>                   // SAFETY: `node` is a non-null node so it is valid by the type invariants.
>                   Ordering::Greater => unsafe { (*node).rb_right },
> -                Ordering::Equal => return NonNull::new(this as _),
> +                // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +                Ordering::Equal => return Some(unsafe { &(*this).value }),
>               }
>           }
>           None
>       }
> 
> -    /// Returns a reference to the value corresponding to the key.
> -    pub fn get(&self, key: &K) -> Option<&V> {
> -        // SAFETY: The `find` return value is a node in the tree, so it is valid.
> -        self.find(key).map(|node| unsafe { &node.as_ref().value })
> -    }
> -
>       /// Returns a mutable reference to the value corresponding to the key.
>       pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
> -        // SAFETY: The `find` return value is a node in the tree, so it is valid.
> -        self.find(key)
> -            .map(|mut node| unsafe { &mut node.as_mut().value })
> +        self.find_mut(key).map(|node| node.into_mut())
>       }
> 
>       /// Removes the node with the given key from the tree.
>       ///
>       /// It returns the node that was removed if one exists, or [`None`] otherwise.
> -    fn remove_node(&mut self, key: &K) -> Option<RBTreeNode<K, V>> {
> -        let mut node = self.find(key)?;
> -
> -        // SAFETY: The `find` return value is a node in the tree, so it is valid.
> -        unsafe { bindings::rb_erase(&mut node.as_mut().links, &mut self.root) };
> -
> -        // INVARIANT: The node is being returned and the caller may free it, however, it was
> -        // removed from the tree. So the invariants still hold.
> -        Some(RBTreeNode {
> -            // SAFETY: The `find` return value was a node in the tree, so it is valid.
> -            node: unsafe { Box::from_raw(node.as_ptr()) },
> -        })
> +    pub fn remove_node(&mut self, key: &K) -> Option<RBTreeNode<K, V>> {
> +        self.find_mut(key).map(OccupiedEntry::remove_node)
>       }
> 
>       /// Removes the node with the given key from the tree.
>       ///
>       /// It returns the value that was removed if one exists, or [`None`] otherwise.
>       pub fn remove(&mut self, key: &K) -> Option<V> {
> -        let node = self.remove_node(key)?;
> -        let RBTreeNode { node } = node;
> -        let Node {
> -            links: _,
> -            key: _,
> -            value,
> -        } = *node;
> -        Some(value)
> +        self.find_mut(key).map(OccupiedEntry::remove)
>       }
> 
>       /// Returns a cursor over the tree nodes based on the given key.
> @@ -1064,3 +1048,167 @@ unsafe impl<K: Send, V: Send> Send for RBTreeNode<K, V> {}
>   // SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
>   // fields, so we use the same Sync condition as would be used for a struct with K and V fields.
>   unsafe impl<K: Sync, V: Sync> Sync for RBTreeNode<K, V> {}
> +
> +impl<K, V> RBTreeNode<K, V> {
> +    /// "Uninitialises" a node.

I think "Drop the key and value, but keep the allocation." might fit
better.

> +    ///
> +    /// It then becomes a reservation that can be re-initialised into a different node (i.e., with
> +    /// a different key and/or value).
> +    ///
> +    /// The existing key and value are dropped in-place as part of this operation, that is, memory
> +    /// may be freed (but only for the key/value; memory for the node itself is kept for reuse).
> +    pub fn into_reservation(self) -> RBTreeNodeReservation<K, V> {
> +        let raw = Box::into_raw(self.node);
> +        let mut ret = RBTreeNodeReservation {
> +            // SAFETY: The pointer came from a valid `Node`, which has the same layout as
> +            // `MaybeUninit<Node>`.
> +            node: unsafe { Box::from_raw(raw as _) },
> +        };
> +        // SAFETY: Although the type is `MaybeUninit<Node>`, we know it has been initialised
> +        // because it came from a `Node`. So it is safe to drop it.
> +        unsafe { core::ptr::drop_in_place(ret.node.as_mut_ptr()) };
> +        ret
> +    }
> +}
> +
> +/// A view into a single entry in a map, which may either be vacant or occupied.
> +///
> +/// This enum is constructed from the [`entry`] method on [`RBTree`].
> +///
> +/// [`entry`]: fn@RBTree::entry
> +pub enum Entry<'a, K, V> {
> +    /// This [`RBTree`] does not have a node with this key.
> +    Vacant(VacantEntry<'a, K, V>),
> +    /// This [`RBTree`] already has a node with this key.
> +    Occupied(OccupiedEntry<'a, K, V>),
> +}
> +
> +/// Like [`Entry`], except that it doesn't have ownership of the key.
> +enum RawEntry<'a, K, V> {
> +    Vacant(RawVacantEntry<'a, K, V>),
> +    Occupied(OccupiedEntry<'a, K, V>),
> +}
> +
> +/// A view into a vacant entry in a [`RBTree`]. It is part of the [`Entry`] enum.
> +pub struct VacantEntry<'a, K, V> {
> +    key: K,
> +    raw: RawVacantEntry<'a, K, V>,
> +}
> +
> +/// Like [`VacantEntry`], but doesn't hold on to the key.
> +struct RawVacantEntry<'a, K, V> {
> +    rbtree: &'a mut RBTree<K, V>,
> +    /// The node that will become the parent of the new node if we insert one.
> +    ///
> +    /// This pointer may be null if the new node becomes the root.
> +    parent: *mut bindings::rb_node,
> +    /// This points to the left-child or right-child field of `parent`. This controls whether the
> +    /// new node will become the left or right child of `parent`.
> +    ///
> +    /// If `parent` is null, then this points at `rbtree.root`.

These constraints should be type invariants.

> +    new_link: *mut *mut bindings::rb_node,
> +}
> +
> +impl<'a, K, V> RawVacantEntry<'a, K, V> {
> +    /// Inserts the given node into the [`RBTree`] at this entry.
> +    ///
> +    /// The `node` must have a key such that inserting it here does not break the ordering of this
> +    /// [`RBTree`].
> +    fn insert(self, node: RBTreeNode<K, V>) -> &'a mut V {
> +        let node = Box::into_raw(node.node);
> +
> +        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
> +        // the node is removed or replaced.
> +        let node_links = unsafe { addr_of_mut!((*node).links) };
> +
> +        // INVARIANT: We are linking in a new node, which is valid. It remains valid because we
> +        // "forgot" it with `Box::into_raw`.
> +        // SAFETY: All pointers are null or valid in an appropriate way.
> +        unsafe { bindings::rb_link_node(node_links, self.parent, self.new_link) };
> +
> +        // SAFETY: All pointers are valid. `node` has just been inserted into the tree.
> +        unsafe { bindings::rb_insert_color(node_links, &mut self.rbtree.root) };
> +
> +        // SAFETY: The node is valid until we remove it from the tree.
> +        unsafe { &mut (*node).value }
> +    }
> +}
> +
> +impl<'a, K, V> VacantEntry<'a, K, V> {
> +    /// Inserts the given node into the [`RBTree`] at this entry.
> +    pub fn insert(self, value: V, reservation: RBTreeNodeReservation<K, V>) -> &'a mut V {
> +        self.raw.insert(reservation.into_node(self.key, value))
> +    }
> +}
> +
> +/// A view into an occupied entry in a [`RBTree`]. It is part of the [`Entry`] enum.
> +pub struct OccupiedEntry<'a, K, V> {
> +    rbtree: &'a mut RBTree<K, V>,
> +    /// The node that this entry corresponds to. Non null.

Must this pointer also be valid? This should be an invariant.

> +    node_links: *mut bindings::rb_node,
> +}
> +
> +impl<'a, K, V> OccupiedEntry<'a, K, V> {
> +    fn node_ptr(&self) -> *mut Node<K, V> {
> +        // SAFETY: All links fields we create are in a `Node<K, V>`.
> +        unsafe { crate::container_of!(self.node_links, Node<K, V>, links) }.cast_mut()
> +    }
> +
> +    /// Gets a reference to the value in the entry.
> +    pub fn get(&self) -> &V {
> +        unsafe { &(*self.node_ptr()).value }

Missing SAFETY comment.

> +    }
> +
> +    /// Gets a mutable reference to the value in the entry.
> +    pub fn get_mut(&mut self) -> &mut V {
> +        unsafe { &mut (*self.node_ptr()).value }

Ditto.

> +    }
> +
> +    /// Converts the entry into a mutable reference to its value.
> +    ///
> +    /// If you need multiple references to the `OccupiedEntry`, see [`self#get_mut`].
> +    pub fn into_mut(self) -> &'a mut V {
> +        unsafe { &mut (*self.node_ptr()).value }

Ditto.

> +    }
> +
> +    /// Remove this entry from the [`RBTree`].
> +    pub fn remove_node(self) -> RBTreeNode<K, V> {
> +        // SAFETY: The node is a node in the tree, so it is valid.
> +        unsafe { bindings::rb_erase(self.node_links, &mut self.rbtree.root) };
> +
> +        // INVARIANT: The node is being returned and the caller may free it, however, it was
> +        // removed from the tree. So the invariants still hold.
> +        RBTreeNode {
> +            // SAFETY: The node was a node in the tree, but we removed it, so we can convert it
> +            // back into a box.
> +            node: unsafe { Box::from_raw(self.node_ptr()) },
> +        }
> +    }
> +
> +    /// Takes the value of the entry out of the map, and returns it.
> +    pub fn remove(self) -> V {
> +        self.remove_node().node.value
> +    }
> +
> +    /// Swap the current node for the provided node.
> +    ///
> +    /// The key of both nodes must be equal.
> +    fn replace(self, node: RBTreeNode<K, V>) -> RBTreeNode<K, V> {
> +        let node = Box::into_raw(node.node);
> +
> +        // SAFETY: `node` is valid at least until we call `Box::from_raw`, which only happens when
> +        // the node is removed or replaced.
> +        let new_node_links = unsafe { addr_of_mut!((*node).links) };
> +
> +        // SAFETY: This updates the pointers so that `new_node_links` is in the tree where
> +        // `self.node_links` used to be.
> +        unsafe {
> +            bindings::rb_replace_node(self.node_links, new_node_links, &mut self.rbtree.root)
> +        };
> +
> +        // SAFETY: Now that we removed this entry from the tree, we can convert the node to a box.

This comment does not explain why `self.node_ptr()` is a valid pointer.

-- 
Cheers,
Benno

> +        let old_node = unsafe { Box::from_raw(self.node_ptr()) };
> +
> +        RBTreeNode { node: old_node }
> +    }
> +}
> 
> --
> 2.44.0.rc0.258.g7320e95886-goog
>