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

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Message-ID: <aHGBQLSzOq6RsqKt@tardis-2.local>
Date: Fri, 11 Jul 2025 14:25:20 -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 08:34:07PM +0200, Benno Lossin wrote:
[...]
> >
> > So all your disagreement is about the "extra safety requirement" part?
> > How about I drop that:
> >
> >     /// Returns a pointer to the underlying atomic `T`.
> >     pub const fn as_ptr(&self) -> *mut T {
> >         self.0.get()
> >     }
> 
> Yes that's what I had in mind.
> 
> > ? I tried to add something additional information:
> >
> > /// Note that non-atomic reads and writes via the returned pointer may
> > /// cause data races if racing with atomic reads and writes per [LKMM].
> >
> > but that seems redundant, because as you said, data races are UB anyway.
> 
> Yeah... I don't think the stdlib docs are too useful on this function:
> 
>     Doing non-atomic reads and writes on the resulting integer can be a data
>     race. This method is mostly useful for FFI, where the function signature
>     may use *mut i32 instead of &AtomicI32.
>     
>     Returning an *mut pointer from a shared reference to this atomic is safe
>     because the atomic types work with interior mutability. All
>     modifications of an atomic change the value through a shared reference,
>     and can do so safely as long as they use atomic operations. Any use of
>     the returned raw pointer requires an unsafe block and still has to
>     uphold the same restriction: operations on it must be atomic.
> 
> You can mention the use of this function for FFI. People might then be
> discouraged from using it for other things where it doesn't make sense.
> 

I'm going to keep it simple at the beginning (i.e. using the one-line
doc comment above). I added it in an issue so that we can revisit it
later:
	
	https://github.com/Rust-for-Linux/linux/issues/1180

For your other feebacks on patch #4, I think they are reasonable and I'm
going to apply them, except I may need an extra review on the doc
comment of Atomic<T> when I have it. Thanks!

Regards,
Boqun

> ---
> Cheers,
> Benno