[RFC PATCH v2 2/4] rust: maple_tree: Add load_rcu()

[Boqun Feng <boqun@kernel.org>]

From: Alice Ryhl <aliceryhl@google.com>

Now that we have a concept of rcu-safe containers, we may add a
load_rcu() method to MapleTree that does not take the spinlock.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/maple_tree.rs | 52 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/rust/kernel/maple_tree.rs b/rust/kernel/maple_tree.rs
index 265d6396a78a..1499191b8935 100644
--- a/rust/kernel/maple_tree.rs
+++ b/rust/kernel/maple_tree.rs
@@ -16,6 +16,10 @@
     alloc::Flags,
     error::to_result,
     prelude::*,
+    sync::rcu::{
+        self,
+        ForeignOwnableRcu, //
+    },
     types::{ForeignOwnable, Opaque},
 };
 
@@ -233,6 +237,54 @@ pub fn erase(&self, index: usize) -> Option<T> {
         unsafe { T::try_from_foreign(ret) }
     }
 
+    /// Load the value at the given index with rcu.
+    ///
+    /// # Examples
+    ///
+    /// Read the value under an rcu read lock. Even if the value is removed, it remains accessible
+    /// for one rcu grace period.
+    ///
+    /// ```ignore
+    /// use kernel::{
+    ///     maple_tree::MapleTree,
+    ///     sync::rcu::{self, RcuBox},
+    /// };
+    ///
+    /// let tree = KBox::pin_init(MapleTree::<RcuBox<i32>>::new(), GFP_KERNEL)?;
+    ///
+    /// let ten = RcuBox::new(10, GFP_KERNEL)?;
+    /// tree.insert(100, ten, GFP_KERNEL)?;
+    ///
+    /// let rcu_read_lock = rcu::Guard::new();
+    /// let ten = tree.load_rcu(100, &rcu_read_lock);
+    /// assert_eq!(ten, Some(&10));
+    ///
+    /// // Even if the value gets removed, we may continue to access it for one rcu grace period.
+    /// tree.erase(100);
+    /// assert_eq!(ten, Some(&10));
+    /// # Ok::<_, Error>(())
+    /// ```
+    #[inline]
+    pub fn load_rcu<'rcu>(
+        &self,
+        index: usize,
+        _rcu: &'rcu rcu::Guard,
+    ) -> Option<T::RcuBorrowed<'rcu>>
+    where
+        T: ForeignOwnableRcu,
+    {
+        // SAFETY: `self.tree` contains a valid maple tree.
+        let ret = unsafe { bindings::mtree_load(self.tree.get(), index) };
+        if ret.is_null() {
+            return None;
+        }
+
+        // SAFETY: If the pointer is not null, then it references a valid instance of `T`. It is
+        // safe to borrow the instance for 'rcu because the signature of this function enforces that
+        // the borrow does not outlive an rcu grace period.
+        Some(unsafe { T::rcu_borrow(ret) })
+    }
+
     /// Lock the internal spinlock.
     #[inline]
     pub fn lock(&self) -> MapleGuard<'_, T> {
-- 
2.50.1 (Apple Git-155)