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

Andreas Hindborg
2025-07-08T08:54:16Z

"Boqun Feng" <boqun.feng@gmail.com> writes:

> 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.
>

You would like to remove `PhantomPinned` to enable noalias? I guess that
makes sense in this case. I'll fix that for next spin.

Best regards,
Andreas Hindborg