linux-kernel - Re: [PATCH v5 03/10] rust: sync: atomic: Add ordering annotation types

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Message-ID: <20250619151050.GN1613376@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 17:10:50 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Boqun Feng <boqun.feng@...il.com>
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>,
	Benno Lossin <lossin@...nel.org>,
	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>,
	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>
Subject: Re: [PATCH v5 03/10] rust: sync: atomic: Add ordering annotation
 types

On Thu, Jun 19, 2025 at 08:00:30AM -0700, Boqun Feng wrote:

> > So given we build locks from atomics, this has to come from somewhere.
> > 
> > The simplest lock -- TAS -- is: rmw.acquire + store.release.
> > 
> > So while plain store.release + load.acquire might not make TSO (although
> > IIRC ARM added variants that do just that in an effort to aid x86
> > emulation); store.release + rmw.acquire must, otherwise we cannot
> > satisfy that unlock+lock.
> 
> Make sense, so something like this in the model should work:
> 
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index d7e7bf13c831..90cb6db6e335 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -27,7 +27,7 @@ include "lock.cat"
>  (* Release Acquire *)
>  let acq-po = [Acquire] ; po ; [M]
>  let po-rel = [M] ; po ; [Release]
> -let po-unlock-lock-po = po ; [UL] ; (po|rf) ; [LKR] ; po
> +let po-unlock-lock-po = po ; (([UL] ; (po|rf) ; [LKR]) | ([Release]; (po;rf); [Acquire & RMW])) ; po
> 
>  (* Fences *)
>  let R4rmb = R \ Noreturn       (* Reads for which rmb works *)
> 

I am forever struggling with cats, but that does look about right :-)

> although I'm not sure whether there will be actual users that use this
> ordering.

include/asm-generic/ticket_spinlock.h comes to mind, as I thing would
kernel/locking/qspinlock.*, no?