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

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

"Benno Lossin" <lossin@kernel.org> writes:

> On Mon Jul 7, 2025 at 3:29 PM CEST, Andreas Hindborg 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>
>
> One nit and a safety comment fix below. (feel free to ignore the nit)
> With the safety comment fixed:
>
> Reviewed-by: Benno Lossin <lossin@kernel.org>
>
>> ---
>>  rust/kernel/sync.rs          |   2 +
>>  rust/kernel/sync/set_once.rs | 125 +++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 127 insertions(+)
>>
>> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
>> index 81e3a806e57e2..13e6bc7fa87ac 100644
>> --- a/rust/kernel/sync.rs
>> +++ b/rust/kernel/sync.rs
>> @@ -18,6 +18,7 @@
>>  mod locked_by;
>>  pub mod poll;
>>  pub mod rcu;
>> +mod set_once;
>
> I would have named this `once`.

So module `once` and struct `SetOnce`? Struct name `Once` would lead
thoughts to `std::sync::Once`, which is a different thing.

>
>>  pub use arc::{Arc, ArcBorrow, UniqueArc};
>>  pub use completion::Completion;
>
>> +    /// Get a reference to the contained object.
>> +    ///
>> +    /// Returns [`None`] if this [`SetOnce`] is empty.
>> +    pub fn as_ref(&self) -> Option<&T> {
>> +        if self.init.load(Acquire) == 2 {
>> +            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
>> +            // contains a valid value.
>
> And the type invariants also ensure that the value of `self.init`
> doesn't change.
>
> So probably
>
>     // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
>     // contains a valid value. They also guarantee that `self.init` doesn't change.
>

Sure 👍


Best regards,
Andreas Hindborg