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

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Message-ID: <87v7o2omqf.fsf@kernel.org>
Date: Tue, 08 Jul 2025 15:06:48 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Benno Lossin" <lossin@...nel.org>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Alex Gaynor"
 <alex.gaynor@...il.com>,  "Boqun Feng" <boqun.feng@...il.com>,  "Gary Guo"
 <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Alice
 Ryhl" <aliceryhl@...gle.com>,  "Masahiro Yamada" <masahiroy@...nel.org>,
  "Nathan Chancellor" <nathan@...nel.org>,  "Luis Chamberlain"
 <mcgrof@...nel.org>,  "Danilo Krummrich" <dakr@...nel.org>,  "Nicolas
 Schier" <nicolas.schier@...ux.dev>,  "Trevor Gross" <tmgross@...ch.edu>,
  "Adam Bratschi-Kaye" <ark.email@...il.com>,
  <rust-for-linux@...r.kernel.org>,  <linux-kernel@...r.kernel.org>,
  <linux-kbuild@...r.kernel.org>,  "Petr Pavlu" <petr.pavlu@...e.com>,
  "Sami Tolvanen" <samitolvanen@...gle.com>,  "Daniel Gomez"
 <da.gomez@...sung.com>,  "Simona Vetter" <simona.vetter@...ll.ch>,  "Greg
 KH" <gregkh@...uxfoundation.org>,  "Fiona Behrens" <me@...enk.dev>,
  "Daniel Almeida" <daniel.almeida@...labora.com>,
  <linux-modules@...r.kernel.org>
Subject: Re: [PATCH v15 1/7] rust: sync: add `SetOnce`

"Benno Lossin" <lossin@...nel.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@...nel.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@...nel.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