linux-kernel - Re: [PATCH v15 1/7] rust: sync: add `SetOnce`

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Message-Id: <DB6JZBUSWGKX.3M3M5TSWPLLFN@kernel.org>
Date: Tue, 08 Jul 2025 11:07:02 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Andreas Hindborg" <a.hindborg@...nel.org>, "Boqun Feng"
 <boqun.feng@...il.com>
Cc: "Alice Ryhl" <aliceryhl@...gle.com>, "Miguel Ojeda" <ojeda@...nel.org>,
 "Alex Gaynor" <alex.gaynor@...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 v15 1/7] rust: sync: add `SetOnce`

On Tue Jul 8, 2025 at 10:54 AM CEST, Andreas Hindborg wrote:
> "Boqun Feng" <boqun.feng@...il.com> writes:
>
>> On Mon, Jul 07, 2025 at 03:38:58PM +0200, Alice Ryhl wrote:
>>> On Mon, Jul 7, 2025 at 3:32 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>> >
>>> > Introduce the `SetOnce` 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>
>>>
>>> LGTM:
>>> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
>>>
>>> > +impl<T> Drop for SetOnce<T> {
>>> > +    fn drop(&mut self) {
>>> > +        if self.init.load(Acquire) == 2 {
>>> > +            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
>>> > +            // contains a valid value. We have exclusive access, as we hold a `mut` reference to
>>> > +            // `self`.
>>> > +            unsafe { drop_in_place(self.value.get()) };
>>>
>>> This load does not need to be Acquire. It can be a Relaxed load or
>>> even an unsynchronized one since the access is exclusive.
>>
>> Right, I think we can do the similar as Revocable here:
>>
>>         if *self.init.get_mut() == 2 { }
>>
>> Further, with my following Benno's suggestion and making `Atomic<T>` an
>> `UnsafeCell<T>:
>>
>> 	https://lore.kernel.org/rust-for-linux/aGhh-TvNOWhkt0JG@Mac.home/
>>
>> compiler can generate a noalias reference here, which allows further
>> optimization.
>>
>
> You would like to remove `PhantomPinned` to enable noalias? I guess that
> makes sense in this case. I'll fix that for next spin.

I think you two are talking about different things. Boqun is saying that
the `Atomic<T>` will use `UnsafeCell` rather than `Opaque`, which will
potentially allow more optimizations.

But you are talking about `SetOnce`, right? I think it makes more sense
for `SetOnce` to use `UnsafeCell<MaybeUninit<T>>` rather than `Opaque`
too. So feel free to change it in the next version.

---
Cheers,
Benno