rust: add rcu abstraction

Based on https://github.com/wedsonaf/linux/commit/02541e65a7e778c0049fed86ae49302bc07abed3

3 files changed, 68 insertions, 0 deletions

diff --git a/rust/helpers.c b/rust/helpers.c
index 92b494ad66a6..c776086017fd 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -29,6 +29,7 @@
 #include <linux/highmem.h>
 #include <linux/mutex.h>
 #include <linux/pci.h>
+#include <linux/rcupdate.h>
 #include <linux/refcount.h>
 #include <linux/sched/signal.h>
 #include <linux/spinlock.h>
@@ -319,6 +320,20 @@ void rust_helper_memcpy_fromio(void *to, const volatile void __iomem *from, long
 EXPORT_SYMBOL_GPL(rust_helper_memcpy_fromio);
 /* end io.h */
 
+/* rcu */
+void rust_helper_rcu_read_lock(void)
+{
+	rcu_read_lock();
+}
+EXPORT_SYMBOL_GPL(rust_helper_rcu_read_lock);
+
+void rust_helper_rcu_read_unlock(void)
+{
+	rcu_read_unlock();
+}
+EXPORT_SYMBOL_GPL(rust_helper_rcu_read_unlock);
+/* end rcu */
+
 /*
  * `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/sync.rs b/rust/kernel/sync.rs
index d219ee518eff..874a45b0c168 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -11,6 +11,7 @@ mod arc;
 mod condvar;
 pub mod lock;
 mod locked_by;
+pub mod rcu;
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
 pub use condvar::CondVar;
diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
new file mode 100644
index 000000000000..1a1c8ea49359
--- /dev/null
+++ b/rust/kernel/sync/rcu.rs
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! RCU support.
+//!
+//! C header: [`include/linux/rcupdate.h`](../../../../include/linux/rcupdate.h)
+
+use crate::bindings;
+use core::marker::PhantomData;
+
+/// Evidence that the RCU read side lock is held on the current thread/CPU.
+///
+/// The type is explicitly not `Send` because this property is per-thread/CPU.
+///
+/// # Invariants
+///
+/// The RCU read side lock is actually held while instances of this guard exist.
+pub struct Guard {
+    _not_send: PhantomData<*mut ()>,
+}
+
+impl Guard {
+    /// Acquires the RCU read side lock and returns a guard.
+    pub fn new() -> Self {
+        // SAFETY: An FFI call with no additional requirements.
+        unsafe { bindings::rcu_read_lock() };
+        // INVARIANT: The RCU read side lock was just acquired above.
+        Self {
+            _not_send: PhantomData,
+        }
+    }
+
+    /// Explicitly releases the RCU read side lock.
+    pub fn unlock(self) {}
+}
+
+impl Default for Guard {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Drop for Guard {
+    fn drop(&mut self) {
+        // SAFETY: By the type invariants, the rcu read side is locked, so it is ok to unlock it.
+        unsafe { bindings::rcu_read_unlock() };
+    }
+}
+
+/// Acquires the RCU read side lock.
+pub fn read_lock() -> Guard {
+    Guard::new()
+}