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From: Andreas Hindborg <a.hindborg@kernel.org>
To: Gary Guo <gary@garyguo.net>, Miguel Ojeda <ojeda@kernel.org>, Boqun Feng
 <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, =?utf-8?Q?Bj=C3=B6rn?=
 Roy Baron
 <bjorn3_gh@protonmail.com>, Benno Lossin <lossin@kernel.org>, Alice
 Ryhl <aliceryhl@google.com>, Trevor Gross <tmgross@umich.edu>, Danilo
 Krummrich <dakr@kernel.org>
Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rust: sync: add lazy initialization methods to SetOnce
In-Reply-To: <DGPTYBO26YBT.3S14I9F5YT1PW@garyguo.net>
References: <20260215-set-once-lazy-v1-1-6f5bd2efda11@kernel.org>
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 <DGPTYBO26YBT.3S14I9F5YT1PW@garyguo.net>
Date: Fri, 27 Feb 2026 20:15:59 +0100
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"Gary Guo" <gary@garyguo.net> writes:

> On Sun Feb 15, 2026 at 8:27 PM GMT, Andreas Hindborg wrote:
>> Add methods to get a reference to the contained value or populate the
>> SetOnce if empty. The new `as_ref_or_populate` method accepts a value
>> directly, while `as_ref_or_populate_with` accepts a fallible closure,
>> allowing for lazy initialization that may fail. Both methods spin-wait
>> if another thread is concurrently initializing the container.
>>
>> Also add `populate_with` which takes a fallible closure and serves as
>> the implementation basis for the other populate methods.
>
> Hi Andreas, in an earlier call I mentioned that I'm working on getting SetOnce
> to work with pin-init, the capability of which I think is a superset of you have
> here.
>
> The API I have is
>
> impl<T> SetOnce<T> {
>     pub fn init<E>(&self, init: impl Init<T, E>) -> Result<&T, InitError<E>>;
>     pub fn pin_init<E>(self, Pin<&Self>, init: impl PinInit<T, E>) -> Result<&T, InitError<E>>;
> }
>
> To achieve what you need with a function, you can simply write:
>
>     set_once.init(pin_init::init_scope(your_fn))
>
> The patch that implement the API is here:
> https://github.com/nbdd0121/linux/commit/4aabdbcf20b11626c253f203745b1d55c37ab2ee
> in tree
> https://github.com/nbdd0121/linux/tree/lazy_revocable_nova_wip/
>
> which I haven't submitted to the list as the user side of this API isn't ready.

I probably evicted that from cache.

It looks like that could replace the `populate` in my patch? We would
still need the synchronization in `as_ref_or_populate`, right? (or
`as_ref_or_init` if you will)


Best regards,
Andreas Hindborg