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

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alice Ryhl" <aliceryhl@google.com> writes:

> 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, that is actually very cool. My rationale was that if a reference
has been shared to another thread of execution, we would need to
synchronize here to see a possible initialization from that other
thread. But I guess it is impossible to end the lifetime of a reference
without doing a synchronization somewhere else.


Best regards,
Andreas Hindborg