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

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Message-ID: <aG7NFizFGeFBSXY-@tardis.local>
Date: Wed, 9 Jul 2025 13:12:06 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Andreas Hindborg <a.hindborg@...nel.org>,
	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 09, 2025 at 08:22:16PM +0200, Benno Lossin wrote:
> 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.
> 

Right, when LTO=y, arm64 use acquire load to implement
READ_ONCE()/atomic_read().

Regards,
Boqun

> ---
> Cheers,
> Benno
>