[PATCH v1 2/3] rust: sync: set_once: Implement Send and Sync

[FUJITA Tomonori <fujita.tomonori@gmail.com>]

Implement Send and Sync for SetOnce<T> to allow it to be used across
thread boundaries.

Send: SetOnce<T> can be transferred across threads when T: Send, as
the contained value is also transferred and will be dropped on the
destination thread.

Sync: SetOnce<T> can be shared across threads when T: Sync, as
as_ref() provides shared references &T and atomic operations ensure
proper synchronization.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/sync/set_once.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/rust/kernel/sync/set_once.rs b/rust/kernel/sync/set_once.rs
index 889d37dfd17f..5894d0dda14e 100644
--- a/rust/kernel/sync/set_once.rs
+++ b/rust/kernel/sync/set_once.rs
@@ -127,3 +127,10 @@ fn drop(&mut self) {
         }
     }
 }
+
+// SAFETY: `SetOnce` can be transferred across thread boundaries iff the data it contains can.
+unsafe impl<T: Send> Send for SetOnce<T> {}
+
+// SAFETY: `SetOnce` synchronises access to the inner value via atomic operations, so it is `Sync`
+// as long as the data it contains is `Sync`.
+unsafe impl<T: Sync> Sync for SetOnce<T> {}
-- 
2.43.0