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

Benno Lossin
2025-07-09T20:22:10Z

On Wed Jul 9, 2025 at 10:12 PM CEST, Boqun Feng wrote:
> 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().

But Andreas was talking about relaxed load vs mutable reference (=
normal unsynchronized write)?

---
Cheers,
Benno