linux-kernel - Re: [PATCHv2 3/4] tools/memory-model: Define effect of Mb tags on RMWs in tools/...

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Date: Mon, 17 Jun 2024 15:43:03 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: paulmck@...nel.org, stern@...land.harvard.edu, parri.andrea@...il.com,
	will@...nel.org, peterz@...radead.org, npiggin@...il.com,
	dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
	akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
	urezki@...il.com, quic_neeraju@...cinc.com, frederic@...nel.org,
	linux-kernel@...r.kernel.org, Viktor Vafeiadis <viktor@...-sws.org>
Subject: Re: [PATCHv2 3/4] tools/memory-model: Define effect of Mb tags on
 RMWs in tools/...

On Wed, Jun 05, 2024 at 09:56:31PM +0200, Jonas Oberhauser wrote:
> 
> 
> Am 6/5/2024 um 6:28 AM schrieb Boqun Feng:
> > On Tue, Jun 04, 2024 at 06:04:40PM +0200, Jonas Oberhauser wrote:
> > > Herd7 transforms successful RMW with Mb tags by inserting smp_mb() fences
> > > around them. We emulate this by considering imaginary po-edges before the
> > > RMW read and before the RMW write, and extending the smp_mb() ordering
> > > rule, which currently only applies to real po edges that would be found
> > > around a really inserted smp_mb(), also to cases of the only imagined po
> > > edges.
> > > 
> > > Reported-by: Viktor Vafeiadis <viktor@...-sws.org>
> > > Suggested-by: Alan Stern <stern@...land.harvard.edu>
> > > Signed-off-by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
> > > ---
> > >   tools/memory-model/linux-kernel.cat | 10 ++++++++++
> > >   1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> > > index adf3c4f41229..d7e7bf13c831 100644
> > > --- a/tools/memory-model/linux-kernel.cat
> > > +++ b/tools/memory-model/linux-kernel.cat
> > > @@ -34,6 +34,16 @@ let R4rmb = R \ Noreturn	(* Reads for which rmb works *)
> > >   let rmb = [R4rmb] ; fencerel(Rmb) ; [R4rmb]
> > >   let wmb = [W] ; fencerel(Wmb) ; [W]
> > >   let mb = ([M] ; fencerel(Mb) ; [M]) |
> > > +	(*
> > > +	 * full-barrier RMWs (successful cmpxchg(), xchg(), etc.) act as
> > > +	 * though there were enclosed by smp_mb().
> > > +	 * The effect of these virtual smp_mb() is formalized by adding
> > > +	 * Mb tags to the read and write of the operation, and providing
> > > +	 * the same ordering as though there were additional po edges
> > > +	 * between the Mb tag and the read resp. write.
> > > +	 *)
> > > +	([M] ; po ; [Mb & R]) |
> > > +	([Mb & W] ; po ; [M]) |
> > 
> > I couldn't help suggestting:
> > 
> > 	([M] ; po ; [Mb & domain(rmw)]) |
> > 	([Mb & range(rmw)] ; po ; [M]) |
> > 
> > , it's a bit more clear to me, but maybe the comment above is good
> > enough?
> 
> Hm, maybe clarity is in the eye of the beholder in this case.
> 
> Actually looking at your suggestion makes me think of smp_store_mb(), which
> although represented as Once;F[Mb] could be (mis)understood also as Mb&W.
> And it indeed does the same thing
>   ([Mb & W] ; po ; [M])
> would suggest.
> 
> (btw I think it is confusing that smp_store_mb is not strictly stronger than
> smp_store_release. Of course there are places where you want a relaxed store
> followed by an mb, but usually the mb versions are strictly stronger.).
> 

May not be a good idea to model smp_store_mb() as a (Mb & W), since the
purpose of smp_store_mb() is for SB pattern synchronization. Maybe it
has a bad name, but I think the intentation of smp_store_mb() is simply
a write + smp_mb(), rather than a MB write.

Regards,
Boqun

> Best wishes,
>   jonas
>