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

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Message-ID: <aHStCX_pxoGog8RF@Mac.home>
Date: Mon, 14 Jul 2025 00:08:57 -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 08:35:25PM +0200, Benno Lossin wrote:
[..]
> >> > +///
> >> > +/// # Guarantees
> >> > +///
> >> > +/// Doing an atomic operation while holding a reference of [`Self`] won't cause a data race,
> >> > +/// this is guaranteed by the safety requirement of [`Self::from_ptr()`] and the extra safety
> >> > +/// requirement of the usage on pointers returned by [`Self::as_ptr()`].
> >> 
> >> I'd rather think we turn this into an invariant that says any operations
> >> on `self.0` through a shared reference is atomic.
> >> 
[...]
> > [...]
> >> > +        let a = self.as_ptr().cast::<T::Repr>();
> >> > +
> >> > +        // SAFETY:
> >> > +        // - For calling the atomic_read*() function:
> >> > +        //   - `a` is a valid pointer for the function per the CAST justification above.
> >> > +        //   - Per the type guarantees, the following atomic operation won't cause data races.
> >> 
> >> Which type guarantees? `Self`?
> >> 
> >
> > The above "# Guarantees" of `Atomic<T>`, but yeah I think it should be
> > "# Invariants".
> 
> Yeah and if we use invariants/guarantees always mention the type that
> they are of and don't assume the reader will "know" :)
> 

Just FYI, I ended dropping the "# Invariants" (or Guarantees) of
`Atomic<T>` because I don't need them any more. Instead I add a
guarantee for Atoimc::as_ptr(): always returns a properly aligned
pointer, which I actually need for calling C functions.

Regards,
Boqun

> ---
> Cheers,
> Benno