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

Boqun Feng
2025-07-09T20:12:10Z

On Wed, Jul 09, 2025 at 08:22:16PM +0200, Benno Lossin wrote:
> On Wed Jul 9, 2025 at 12:34 PM CEST, Andreas Hindborg wrote:
> > "Benno Lossin" <lossin@kernel.org> writes:
> >> On Tue Jul 8, 2025 at 10:54 AM CEST, Andreas Hindborg wrote:
> >>> "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 { }
> >
> > 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().

Regards,
Boqun

> ---
> Cheers,
> Benno
>