linux-kernel - Re: [PATCH v14 1/7] rust: sync: add `OnceLock`

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Message-ID: <87y0t5tf56.fsf@kernel.org>
Date: Thu, 03 Jul 2025 18:25:57 +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 v14 1/7] rust: sync: add `OnceLock`

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

> On Thu Jul 3, 2025 at 11:03 AM CEST, Andreas Hindborg wrote:
>> "Benno Lossin" <lossin@...nel.org> writes:
>>> On Wed Jul 2, 2025 at 3:18 PM CEST, Andreas Hindborg wrote:

[...]

>>>> +            Some(unsafe { &*self.value.get() })
>>>> +        } else {
>>>> +            None
>>>> +        }
>>>> +    }
>>>> +
>>>> +    /// Populate the [`OnceLock`].
>>>> +    ///
>>>> +    /// Returns `true` if the [`OnceLock`] was successfully populated.
>>>> +    pub fn populate(&self, value: T) -> bool {
>>>> +        // INVARIANT: We obtain exclusive access to the contained allocation and write 1 to
>>>> +        // `init`.
>>>> +        if let Ok(0) = self.init.cmpxchg(0, 1, Acquire) {
>>>> +            // SAFETY: We obtained exclusive access to the contained object.
>>>> +            unsafe { core::ptr::write(self.value.get(), value) };
>>>> +            // INVARIANT: We release our exclusive access and transition the object to shared
>>>> +            // access.
>>>> +            self.init.store(2, Release);
>>>> +            true
>>>> +        } else {
>>>> +            false
>>>> +        }
>>>> +    }
>>>> +}
>>>> +
>>>> +impl<T: Copy> OnceLock<T> {
>>>> +    /// Get a copy of the contained object.
>>>> +    ///
>>>> +    /// Returns [`None`] if the [`OnceLock`] is empty.
>>>> +    pub fn copy(&self) -> Option<T> {
>>>> +        if self.init.load(Acquire) == 2 {
>>>> +            // SAFETY: As determined by the load above, the object is ready for shared access.
>>>> +            Some(unsafe { *self.value.get() })
>>>> +        } else {
>>>> +            None
>>>> +        }
>>>
>>> The impl can just be:
>>>
>>>     self.as_ref().copied()
>>
>> Nice. I was thinking of dropping this method and just have callers do
>>
>>  my_once_lock.as_ref().map(|v| v.copied())
>>
>> What do you think?
>
> There is `Option::copied`, so no need for the `.map` call.

Cool.

> I don't
> really have a preference, if users always want to access it by-value,
> then we should have `copy`.

But should it be `copy` or `copied` like `Option`?


Best regards,
Andreas Hindborg