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

[Benno Lossin]

On Tue Jul 8, 2025 at 10:54 AM CEST, Andreas Hindborg wrote:
> "Boqun Feng" <boqun.feng@xxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>
>>> LGTM:
>>> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
>>>
>>> > +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@xxxxxxxx/
>>
>> 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.

I think you two are talking about different things. Boqun is saying that
the `Atomic<T>` will use `UnsafeCell` rather than `Opaque`, which will
potentially allow more optimizations.

But you are talking about `SetOnce`, right? I think it makes more sense
for `SetOnce` to use `UnsafeCell<MaybeUninit<T>>` rather than `Opaque`
too. So feel free to change it in the next version.

---
Cheers,
Benno