On Tue Jul 8, 2025 at 10:54 AM CEST, Andreas Hindborg wrote: > "Boqun Feng" <[email protected]> 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 <[email protected]> >>> 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 <[email protected]> >>> >>> LGTM: >>> Reviewed-by: Alice Ryhl <[email protected]> >>> >>> > +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/[email protected]/ >> >> 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
