linux-kernel - Re: [PATCH v6 6/9] rust: sync: atomic: Add the framework of arithmetic operations

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Message-ID: <aHFMwB-aL7Lj_twN@tardis-2.local>
Date: Fri, 11 Jul 2025 10:41:20 -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 6/9] rust: sync: atomic: Add the framework of
 arithmetic operations

On Fri, Jul 11, 2025 at 07:39:15AM -0700, Boqun Feng wrote:
[...]
> > > ---
> > >  rust/kernel/sync/atomic.rs         |  18 +++++
> > >  rust/kernel/sync/atomic/generic.rs | 108 +++++++++++++++++++++++++++++
> > >  2 files changed, 126 insertions(+)
> > 
> > I think it's better to name this trait `AtomicAdd` and make it generic:
> > 
> >     pub unsafe trait AtomicAdd<Rhs = Self>: AllowAtomic {
> >         fn rhs_into_repr(rhs: Rhs) -> Self::Repr;
> >     }
> > 
> > `sub` and `fetch_sub` can be added using a similar trait.
> > 
> 
> Seems a good idea, I will see what I can do. Thanks!
> 
> > The generic allows you to implement it multiple times with different
> > meanings, for example:
> > 
> >     pub struct Nanos(u64);
> >     pub struct Micros(u64);
> >     pub struct Millis(u64);
> > 
> >     impl AllowAtomic for Nanos {
> >         type Repr = i64;
> >     }
> > 
> >     impl AtomicAdd<Millis> for Nanos {
> >         fn rhs_into_repr(rhs: Millis) -> i64 {
> >             transmute(rhs.0 * 1000_000)
> 
> We probably want to use `as` in real code?
> 
> >         }
> >     }
> > 
> >     impl AtomicAdd<Micros> for Nanos {
> >         fn rhs_into_repr(rhs: Micros) -> i64 {
> >             transmute(rhs.0 * 1000)
> >         }
> >     }
> > 
> >     impl AtomicAdd<Nanos> for Nanos {
> >         fn rhs_into_repr(rhs: Nanos) -> i64 {
> >             transmute(rhs.0)
> >         }
> >     }
> > 
> > For the safety requirement on the `AtomicAdd` trait, we might just
> > require bi-directional transmutability... Or can you imagine a case
> > where that is not guaranteed, but a weaker form is?
> 
> I have a case that I don't think it's that useful, but it's similar to
> the `Micros` and `Millis` above, an `Even<T>` where `Even<i32>` is a

Oh I mis-read, it's not similar to `Micros` or `Millis`, but still,
`Even<i32>` itself should have the point.

> `i32` but it's always an even number ;-) So transmute<i32, Even<i32>>()
> is not always sound. Maybe we could add a "TODO" in the safety section
> of `AtomicAdd`, and revisit this later? Like:
> 
> /// (in # Safety)
> /// TODO: The safety requirement may be tightened to bi-directional
> /// transmutability. 
> 
> And maybe also add the `Even` example there?
> 
> Thoughts?
> 

Or since we are going to use fine-grained traits, it's actually easy to
define the safety requirement of `AtomicAdd` (instead of
`AllowAtomicArithmetic) now:

    /// # Safety
    ///
    /// For a `T: AtomicAdd<Rhs>`, the addition result of a valid bit
    /// pattern of `T` and a `T::Repr` convert from `rhs_into_repr()` should
    /// be a valid bit pattern of `T`.
    pub unsafe trait AtomicAdd<Rhs = Self>: AllowAtomic {
       fn rhs_into_repr(rhs: Rhs) -> Self::Repr;
    }

Thoughts?

Regards,
Boqun