linux-kernel - Re: [PATCH v6 4/9] rust: sync: atomic: Add generic atomics

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Message-ID: <aHEQKBT68xvqIIjW@Mac.home>
Date: Fri, 11 Jul 2025 06:22:48 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	lkmm@...ts.linux.dev, linux-arch@...r.kernel.org,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Lyude Paul <lyude@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Mitchell Levy <levymitchell0@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH v6 4/9] rust: sync: atomic: Add generic atomics

On Fri, Jul 11, 2025 at 10:03:07AM +0200, Benno Lossin wrote:
[...]
> > +
> > +    /// Returns a pointer to the underlying atomic variable.
> > +    ///
> > +    /// Extra safety requirement on using the return pointer: the operations done via the pointer
> > +    /// cannot cause data races defined by [`LKMM`].
> 
> I don't think this is correct. I could create an atomic and then share
> it with the C side via this function, since I have exclusive access, the
> writes to this pointer don't need to be atomic.
> 

that's why it says "the operations done via the pointer cannot cause
data races .." instead of saying "it must be atomic".

> We also don't document additional postconditions like this... If you

Please see how Rust std document their `as_ptr()`:

	https://doc.rust-lang.org/std/sync/atomic/struct.AtomicI32.html#method.as_ptr

It mentions that "Doing non-atomic reads and writes on the resulting
integer can be a data race." (although the document is a bit out of
date, since non-atomic read and atomic read are no longer data race now,
see [1])

I think we can use the similar document structure here: providing more
safety requirement on the returning pointers, and...

> really would have to do it like this (which you shouldn't given the
> example above), you would have to make this function `unsafe`, otherwise
> there is no way to ensure that people adhere to it (since it isn't part
> of the safety docs).
> 

...since dereferencing pointers is always `unsafe`, users need to avoid
data races anyway, hence this is just additional information that helps
reasoning.

Regards,
Boqun

> > +    ///
> > +    /// [`LKMM`]: srctree/tools/memory-model
> > +    pub const fn as_ptr(&self) -> *mut T {
> > +        self.0.get()
> > +    }
[...]