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

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Jun 19, 2025 at 05:10:50PM +0200, Peter Zijlstra wrote:
> 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?
> 

Ah, right. Although I thought users outside lock implementation would be
nice, but you're right, we do have users. Previously our reasoning for
correctness of this particular locking ordering kinda depends on
per-architecture memory model reasoning, so modeling this does make
sense.

Regards,
Boqun

>