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

Boqun Feng
2025-07-07T15:13:29Z

On Mon, Jul 07, 2025 at 03:38:58PM +0200, Alice Ryhl wrote:
> 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, I think we can do the similar as Revocable here:

if *self.init.get_mut() == 2 { }

Further, with my following Benno's suggestion and making `Atomic<T>` an
`UnsafeCell<T>:

https://lore.kernel.org/rust-for-linux/aGhh-TvNOWhkt0JG@Mac.home/

compiler can generate a noalias reference here, which allows further
optimization.

Regards,
Boqun

>
> Alice