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().

Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Acked-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Onur Özkan <work@onurozkan.dev>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260805145949.938505-5-phasta@kernel.org
[ Formatted documentation. Sorted tags. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Philipp Stanner

rust/kernel/sync/rcu.rs

@@ -50,3 +50,23 @@ impl Drop for Guard {
 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() };
+}