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

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Message-ID: <CAH5fLgj6mfkvnOCO=EQSz9pyN5OFRF3z3yGfyZHkLzqxjsPJ7A@mail.gmail.com>
Date: Tue, 8 Jul 2025 11:00:30 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
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 v15 1/7] rust: sync: add `SetOnce`

On Tue, Jul 8, 2025 at 10:48 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> "Alice Ryhl" <aliceryhl@...gle.com> writes:
>
> > 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, that is actually very cool. My rationale was that if a reference
> has been shared to another thread of execution, we would need to
> synchronize here to see a possible initialization from that other
> thread. But I guess it is impossible to end the lifetime of a reference
> without doing a synchronization somewhere else.

Yup, a mutable reference generally implies synchronization.

Alice