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

[Gary Guo <gary@garyguo.net>]

On Fri, 12 Dec 2025 08:09:18 +0900
FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:

> 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> {}

This needs to be `T: Send + Sync` as `T` can be destroyed on a
different thread that creates it.

Best,
Gary