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

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Message-ID: <CAH5fLgiOK-zA0qT1dis-BsrEAk96R+E8e_F_361w7kCq_uxn7g@mail.gmail.com>
Date: Wed, 2 Jul 2025 17:27:41 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Andreas Hindborg <a.hindborg@...nel.org>, 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>, 
	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 Wed, Jul 2, 2025 at 5:07 PM Benno Lossin <lossin@...nel.org> wrote:
>
> 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()
>
> Would it make sense for this function to take `self` instead & we make
> the `OnceLock` also `Copy` if `T: Copy`? Maybe not...

Atomics are not Copy.

Alice