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

Alice Ryhl
2025-07-08T09:00:44Z

On Tue, Jul 8, 2025 at 10:48 AM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
> "Alice Ryhl" <aliceryhl@google.com> writes:
>
> > 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.
>
> Right, that is actually very cool. My rationale was that if a reference
> has been shared to another thread of execution, we would need to
> synchronize here to see a possible initialization from that other
> thread. But I guess it is impossible to end the lifetime of a reference
> without doing a synchronization somewhere else.

Yup, a mutable reference generally implies synchronization.

Alice