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

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Message-ID: <aG7ZhTaZRL9ZpyP5@tardis.local>
Date: Wed, 9 Jul 2025 14:05:09 -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 10:22:04PM +0200, Benno Lossin wrote:
[...]
> >> >>>>> > +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().
> 
> But Andreas was talking about relaxed load vs mutable reference (=
> normal unsynchronized write)?
> 

No, I think it was a relaxed load (self.init.load(Relaxed)) vs a normal
unsynchronized *load* (*self.init.get_mut()). Yes, there is a mutable
reference, but we never use it for write.

Regards,
Boqun

> ---
> Cheers,
> Benno
>