Re: [PATCH v2] rust: sync: set_once: Implement Send and Sync

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Dec 16, 2025 at 09:09:01AM +0900, FUJITA Tomonori 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. Since the inner T may be dropped on any
> thread, we also require T: Send.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
> v2:
>   - update the `Sync` impl to require `T: Send + Sync` with the comment adjusted
> v1: https://lore.kernel.org/rust-for-linux/20251211230919.1303926-2-fujita.tomonori@gmail.com/
> 
>  rust/kernel/sync/set_once.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/rust/kernel/sync/set_once.rs b/rust/kernel/sync/set_once.rs
> index bdba601807d8..139cef05e935 100644
> --- a/rust/kernel/sync/set_once.rs
> +++ b/rust/kernel/sync/set_once.rs
> @@ -123,3 +123,11 @@ 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 shared references are safe when `T: Sync`. Since the inner `T` may be dropped
> +// on any thread, we also require `T: Send`.
> +unsafe impl<T: Send + Sync> Sync for SetOnce<T> {}

The reason you need Sync is because &SetOnce<T> allows you to access &T.
It's not because atomic operations are used.

Alice