Re: [PATCH v15 1/7] rust: sync: add `SetOnce`

Alice Ryhl
2025-07-07T13:39:12Z

On Mon, Jul 7, 2025 at 3:32 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
> Introduce the `SetOnce` type, a container that can only be written once.
> The container uses an internal atomic to synchronize writes to the internal
> value.
>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

LGTM:
Reviewed-by: Alice Ryhl <aliceryhl@google.com>

> +impl<T> Drop for SetOnce<T> {
> + fn drop(&mut self) {
> + if self.init.load(Acquire) == 2 {
> + // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
> + // contains a valid value. We have exclusive access, as we hold a `mut` reference to
> + // `self`.
> + unsafe { drop_in_place(self.value.get()) };

This load does not need to be Acquire. It can be a Relaxed load or
even an unsynchronized one since the access is exclusive.

Alice