rust: add rcu abstraction

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

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Tested-by: Fabien Parent <fabien.parent@linaro.org>
Link: https://lore.kernel.org/r/20241219170425.12036-5-dakr@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 48 insertions, 0 deletions

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 1eab7ebf25fd..0654008198b2 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -12,6 +12,7 @@ mod condvar;
 pub mod lock;
 mod locked_by;
 pub mod poll;
+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..b51d9150ffe2
--- /dev/null
+++ b/rust/kernel/sync/rcu.rs
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! RCU support.
+//!
+//! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h)
+
+use crate::{bindings, types::NotThreadSafe};
+
+/// 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(NotThreadSafe);
+
+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(NotThreadSafe)
+    }
+
+    /// 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()
+}