rust: rbtree: add red-black tree implementation backed by the C version

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>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
Link: https://lore.kernel.org/r/20240822-b4-rbtree-v12-1-014561758a57@google.com
[ Updated link to docs.kernel.org. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/helpers/helpers.c

@@ -15,6 +15,7 @@
 #include "kunit.c"
 #include "mutex.c"
 #include "page.c"
+#include "rbtree.c"
 #include "refcount.c"
 #include "signal.c"
 #include "slab.c"

rust/helpers/rbtree.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/rbtree.h>
+
+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);
+}

rust/kernel/lib.rs

@@ -44,6 +44,7 @@
 pub mod page;
 pub mod prelude;
 pub mod print;
+pub mod rbtree;
 mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;

rust/kernel/rbtree.rs

+// SPDX-License-Identifier: GPL-2.0
+
+//! Red-black trees.
+//!
+//! C header: [`include/linux/rbtree.h`](srctree/include/linux/rbtree.h)
+//!
+//! Reference: <https://docs.kernel.org/core-api/rbtree.html>
+
+use crate::{alloc::Flags, bindings, container_of, error::Result, prelude::*};
+use alloc::boxed::Box;
+use core::{
+    cmp::{Ord, Ordering},
+    marker::PhantomData,
+    mem::MaybeUninit,
+    ptr::{addr_of_mut, NonNull},
+};
+
+/// A red-black tree with owned nodes.
+///
+/// It is backed by the kernel C red-black trees.
+///
+/// # 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::{alloc::flags, rbtree::{RBTree, RBTreeNode, RBTreeNodeReservation}};
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(20, 200, flags::GFP_KERNEL)?;
+/// tree.try_create_and_insert(10, 100, flags::GFP_KERNEL)?;
+/// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
+///
+/// // 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, flags::GFP_KERNEL)?;
+///
+/// // 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::{alloc::flags, rbtree::{RBTree, RBTreeNode}, sync::SpinLock};
+///
+/// fn insert_test(tree: &SpinLock<RBTree<u32, u32>>) -> Result {
+///     // Pre-allocate node. This may fail (as it allocates memory).
+///     let node = RBTreeNode::new(10, 100, flags::GFP_KERNEL)?;
+///
+///     // 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::{alloc::flags, rbtree::{RBTree, RBTreeNodeReservation}};
+///
+/// // Create a new tree.
+/// let mut tree = RBTree::new();
+///
+/// // Insert three elements.
+/// tree.try_create_and_insert(20, 200, flags::GFP_KERNEL)?;
+/// tree.try_create_and_insert(10, 100, flags::GFP_KERNEL)?;
+/// tree.try_create_and_insert(30, 300, flags::GFP_KERNEL)?;
+///
+/// // 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(&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);
+/// }
+///
+/// // Create a preallocated reservation that we can re-use later.
+/// let reservation = RBTreeNodeReservation::new(flags::GFP_KERNEL)?;
+///
+/// // 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>(())
+/// ```
+///
+/// # 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.
+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,
+        }
+    }
+}
+
+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,
+        flags: Flags,
+    ) -> Result<Option<RBTreeNode<K, V>>> {
+        Ok(self.insert(RBTreeNode::new(key, value, flags)?))
+    }
+
+    /// 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, RBTreeNode { node }: RBTreeNode<K, V>) -> Option<RBTreeNode<K, V>> {
+        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) };
+
+        // The parameters of `bindings::rb_link_node` are as follows:
+        // - `node`: A pointer to an uninitialized node being inserted.
+        // - `parent`: A pointer to an existing node in the tree. One of its child pointers must be
+        //          null, and `node` will become a child of `parent` by replacing that child pointer
+        //          with a pointer to `node`.
+        // - `rb_link`: A pointer to either the left-child or right-child field of `parent`. This
+        //          specifies which child of `parent` should hold `node` after this call. The
+        //          value of `*rb_link` must be null before the call to `rb_link_node`. If the
+        //          red/black tree is empty, then it’s also possible for `parent` to be null. In
+        //          this case, `rb_link` is a pointer to the `root` field of the red/black tree.
+        //
+        // We will traverse the tree looking for a node that has a null pointer as its child,
+        // representing an empty subtree where we can insert our new node. We need to make sure
+        // that we preserve the ordering of the nodes in the tree. In each iteration of the loop
+        // we store `parent` and `child_field_of_parent`, and the new `node` will go somewhere
+        // in the subtree of `parent` that `child_field_of_parent` points at. Once
+        // we find an empty subtree, we can insert the new node using `rb_link_node`.
+        let mut parent = core::ptr::null_mut();
+        let mut child_field_of_parent: &mut *mut bindings::rb_node = &mut self.root.rb_node;
+        while !child_field_of_parent.is_null() {
+            parent = *child_field_of_parent;
+
+            // We need to determine whether `node` should be the left or right child of `parent`,
+            // so we will compare with the `key` field of `parent` a.k.a. `this` below.
+            //
+            // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
+            // point to the links field of `Node<K, V>` objects.
+            let this = unsafe { container_of!(parent, Node<K, V>, links) };
+
+            // 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) } {
+                // We would like `node` to be the left child of `parent`.  Move to this child to check
+                // whether we can use it, or continue searching, at the next iteration.
+                //
+                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
+                Ordering::Less => child_field_of_parent = unsafe { &mut (*parent).rb_left },
+                // We would like `node` to be the right child of `parent`.  Move to this child to check
+                // whether we can use it, or continue searching, at the next iteration.
+                //
+                // SAFETY: `parent` is a non-null node so it is valid by the type invariants.
+                Ordering::Greater => child_field_of_parent = unsafe { &mut (*parent).rb_right },
+                Ordering::Equal => {
+                    // There is an existing node in the tree with this key, and that node is
+                    // `parent`. Thus, we are replacing parent with a new node.
+                    //
+                    // 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.
+                    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.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 (`*child_field_of_parent` is null, but `child_field_of_parent` is a
+        // mutable reference).
+        unsafe { bindings::rb_link_node(node_links, parent, child_field_of_parent) };
+
+        // 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: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
+            // point to the links field of `Node<K, V>` objects.
+            let this = unsafe { container_of!(node, 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.cast_mut()),
+            }
+        }
+        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> {
+        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 { 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.cast_mut())) };
+        }
+    }
+}
+
+/// 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 ([`RBTreeNodeReservation::new`]).
+pub struct RBTreeNodeReservation<K, V> {
+    node: Box<MaybeUninit<Node<K, V>>>,
+}
+
+impl<K, V> RBTreeNodeReservation<K, V> {
+    /// Allocates memory for a node to be eventually initialised and inserted into the tree via a
+    /// call to [`RBTree::insert`].
+    pub fn new(flags: Flags) -> Result<RBTreeNodeReservation<K, V>> {
+        Ok(RBTreeNodeReservation {
+            node: <Box<_> as BoxExt<_>>::new_uninit(flags)?,
+        })
+    }
+}
+
+// SAFETY: This doesn't actually contain K or V, and is just a memory allocation. Those can always
+// be moved across threads.
+unsafe impl<K, V> Send for RBTreeNodeReservation<K, V> {}
+
+// SAFETY: This doesn't actually contain K or V, and is just a memory allocation.
+unsafe impl<K, V> 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(self, key: K, value: V) -> RBTreeNode<K, V> {
+        let node = Box::write(
+            self.node,
+            Node {
+                key,
+                value,
+                links: bindings::rb_node::default(),
+            },
+        );
+        RBTreeNode { node }
+    }
+}
+
+/// 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>>,
+}
+
+impl<K, V> RBTreeNode<K, V> {
+    /// Allocates and initialises a node that can be inserted into the tree via
+    /// [`RBTree::insert`].
+    pub fn new(key: K, value: V, flags: Flags) -> Result<RBTreeNode<K, V>> {
+        Ok(RBTreeNodeReservation::new(flags)?.into_node(key, value))
+    }
+}
+
+// SAFETY: If K and V can be sent across threads, then it's also okay to send [`RBTreeNode`] across
+// threads.
+unsafe impl<K: Send, V: Send> Send for RBTreeNode<K, V> {}
+
+// SAFETY: If K and V can be accessed without synchronization, then it's also okay to access
+// [`RBTreeNode`] without synchronization.
+unsafe impl<K: Sync, V: Sync> Sync for RBTreeNode<K, V> {}
+
+struct Node<K, V> {
+    links: bindings::rb_node,
+    key: K,
+    value: V,
+}