[RFC v2 12/13] rust: add rcu abstraction

[Boqun Feng <boqun.feng@xxxxxxxxx>]

From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>

Add a simple abstraction to guard critical code sections with an rcu
read lock.

Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
---
 rust/helpers/helpers.c  |  1 +
 rust/helpers/rcu.c      | 13 +++++++++++
 rust/kernel/sync.rs     |  1 +
 rust/kernel/sync/rcu.rs | 52 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 67 insertions(+)
 create mode 100644 rust/helpers/rcu.c
 create mode 100644 rust/kernel/sync/rcu.rs

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index f4a94833b29d..65951245879f 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -18,6 +18,7 @@
 #include "mutex.c"
 #include "page.c"
 #include "rbtree.c"
+#include "rcu.c"
 #include "refcount.c"
 #include "signal.c"
 #include "slab.c"
diff --git a/rust/helpers/rcu.c b/rust/helpers/rcu.c
new file mode 100644
index 000000000000..f1cec6583513
--- /dev/null
+++ b/rust/helpers/rcu.c
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/rcupdate.h>
+
+void rust_helper_rcu_read_lock(void)
+{
+	rcu_read_lock();
+}
+
+void rust_helper_rcu_read_unlock(void)
+{
+	rcu_read_unlock();
+}
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 0d0b19441ae8..f5a413e1ce30 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -13,6 +13,7 @@
 mod condvar;
 pub mod lock;
 mod locked_by;
+pub mod rcu;
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
 pub use condvar::{new_condvar, CondVar, CondVarTimeoutResult};
diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
new file mode 100644
index 000000000000..5a35495f69a4
--- /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`](srctree/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()
+}
-- 
2.45.2