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

Boqun Feng
2025-07-09T21:05:12Z

On Wed, Jul 09, 2025 at 10:22:04PM +0200, Benno Lossin wrote:
[...]
> >> >>>>> > +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 { }
> >> >
> >> > Ok, now I got it. You are saying I don't need to use the atomic load
> >> > method, because I have mutable access. Sounds good.
> >> >
> >> > But I guess a relaxed load and access through a mutable reference should
> >> > result in the same code generation on most (all?) platforms?
> >>
> >> AFAIK it is not the same on arm.
> >>
> >
> > Right, when LTO=y, arm64 use acquire load to implement
> > READ_ONCE()/atomic_read().
>
> But Andreas was talking about relaxed load vs mutable reference (=
> normal unsynchronized write)?
>

No, I think it was a relaxed load (self.init.load(Relaxed)) vs a normal
unsynchronized *load* (*self.init.get_mut()). Yes, there is a mutable
reference, but we never use it for write.

Regards,
Boqun

> ---
> Cheers,
> Benno
>