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

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Message-Id: <DB2PMJKCP2K6.3SRFFDDXL3CV1@kernel.org>
Date: Thu, 03 Jul 2025 22:41:47 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Andreas Hindborg" <a.hindborg@...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`

On Thu Jul 3, 2025 at 6:25 PM CEST, Andreas Hindborg wrote:
> "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:
>>>>> +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`?

I'd say `copy`. With `copied` I'm thinking of something that turns
`OnceLock<&T>` into `OnceLock<T>`, which is weird...

---
Cheers,
Benno