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

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Message-Id: <DB7QEZNTH2SB.SSRG5H7TXZZT@kernel.org>
Date: Wed, 09 Jul 2025 20:22:16 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Andreas Hindborg" <a.hindborg@...nel.org>
Cc: "Boqun Feng" <boqun.feng@...il.com>, "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 Wed Jul 9, 2025 at 12:34 PM CEST, Andreas Hindborg wrote:
> "Benno Lossin" <lossin@...nel.org> writes:
>> 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 { }
>
> Ok, now I got it. You are saying I don't need to use the atomic load
> method, because I have mutable access. Sounds good.
>
> But I guess a relaxed load and access through a mutable reference should
> result in the same code generation on most (all?) platforms?

AFAIK it is not the same on arm.

---
Cheers,
Benno