[PATCH v9 3/5] rust: sync: Add abstraction for rcu_barrier()

rcu_barrier() is a frequently used C function which is always safe to be
called.

Add a safe abstraction for rcu_barrier().

Signed-off-by: Philipp Stanner <[email protected]>
Tested-by: Daniel Almeida <[email protected]>
---
 rust/kernel/sync/rcu.rs | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
index a32bef6e490b..7031ca5d2473 100644
--- a/rust/kernel/sync/rcu.rs
+++ b/rust/kernel/sync/rcu.rs
@@ -50,3 +50,23 @@ fn drop(&mut self) {
 pub fn read_lock() -> Guard {
     Guard::new()
 }
+
+/// Wait until all in-flight call_rcu() callbacks complete.
+///
+/// Note that this primitive does not necessarily wait for an RCU grace period
+/// to complete.  For example, if there are no RCU callbacks queued anywhere
+/// in the system, then rcu_barrier() is within its rights to return
+/// immediately, without waiting for anything, much less an RCU grace period.
+/// In fact, rcu_barrier() will normally not result in any RCU grace periods
+/// beyond those that were already destined to be executed.
+///
+/// In kernels built with CONFIG_RCU_LAZY=y, this function also hurries all
+/// pending lazy RCU callbacks.
+///
+/// Note that this is one of the RCU primitives which must not be called in
+/// atomic context.
+#[inline]
+pub fn rcu_barrier() {
+    // SAFETY: `rcu_barrier()` is always safe to be called. It just might wait for a grace period.
+    unsafe { bindings::rcu_barrier() };
+}
-- 
2.55.0