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

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Message-ID: <87h5ztvhjp.fsf@kernel.org>
Date: Thu, 03 Jul 2025 09:51:06 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
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>,
  "Masahiro Yamada" <masahiroy@...nel.org>,  "Nathan Chancellor"
 <nathan@...nel.org>,  "Luis Chamberlain" <mcgrof@...nel.org>,  "Danilo
 Krummrich" <dakr@...nel.org>,  "Benno Lossin" <lossin@...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`

"Alice Ryhl" <aliceryhl@...gle.com> writes:

> On Wed, Jul 2, 2025 at 3:54 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>
>> "Alice Ryhl" <aliceryhl@...gle.com> writes:
>>
>> > On Wed, Jul 2, 2025 at 3:19 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>> >>
>> >> Introduce the `OnceLock` 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>
>> >
>> > This type provides no way to wait for initialization to finish if it's
>> > ongoing. Do you not need that?
>>
>> I don't, and in my use case it would cause a deadlock to wait. Anyway,
>> it might be useful to others. Would you add it now, or wait for a user?
>
> Waiting would require additional fields so it should probably be a
> different type.

That depends on the kind of waiting. If we do unfair waiting, with
spinning, we can have unlimited waiters with this type. We can also use
the remaining 29 bits of the atomic to encode ordering to get fair
queuing for spinning waiters.

Putting waiters to sleep would require more fields.

> It's more that we probably want the OnceLock name for
> that other type for consistency with stdlib, so perhaps this should be
> renamed? The name could be SetOnce or similar.

The feature set is very similar to `std::sync::OnceLock`, that is why I
picked that name. We can expand this to allow resetting without too much
effort, and we can do fair waiting with spinning for a reasonable amount
of waiters.

But I am also OK with changing the name to `SetOnce` if we envision a
`OnceLock` with thread sleep blocking at some point.


Best regards,
Andreas Hindborg