Date:   Sun, 29 Jan 2023 15:09:00 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     Andrea Parri <parri.andrea@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
        will@kernel.org, peterz@infradead.org, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
        urezki@gmail.com, quic_neeraju@quicinc.com, frederic@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po
Message-ID: <20230129230900.GK2948950@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
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On Sun, Jan 29, 2023 at 01:43:53PM -0800, Boqun Feng wrote:
> On Sun, Jan 29, 2023 at 10:44:03AM -0800, Paul E. McKenney wrote:
> > On Sun, Jan 29, 2023 at 06:28:27PM +0100, Andrea Parri wrote:
> > > > Why can't P3's spin_lock() read from that initial write?
> > > 
> > > Mmh, sounds like you want to play with something like below?
> > > 
> > >   Andrea
> > > 
> > > diff --git a/tools/memory-model/lock.cat b/tools/memory-model/lock.cat
> > > index 6b52f365d73ac..20c3af4511255 100644
> > > --- a/tools/memory-model/lock.cat
> > > +++ b/tools/memory-model/lock.cat
> > > @@ -74,7 +74,6 @@ flag ~empty UL \ range(critical) as unmatched-unlock
> > >  
> > >  (* Allow up to one unmatched LKW per location; more must deadlock *)
> > >  let UNMATCHED-LKW = LKW \ domain(critical)
> > > -empty ([UNMATCHED-LKW] ; loc ; [UNMATCHED-LKW]) \ id as unmatched-locks
> > >  
> > >  (* rfi for LF events: link each LKW to the LF events in its critical section *)
> > >  let rfi-lf = ([LKW] ; po-loc ; [LF]) \ ([LKW] ; po-loc ; [UL] ; po-loc)
> > > @@ -120,8 +119,7 @@ let rf-ru = rfe-ru | rfi-ru
> > >  let rf = rf | rf-lf | rf-ru
> > >  
> > >  (* Generate all co relations, including LKW events but not UL *)
> > > -let co0 = co0 | ([IW] ; loc ; [LKW]) |
> > > -	(([LKW] ; loc ; [UNMATCHED-LKW]) \ [UNMATCHED-LKW])
> > > +let co0 = co0 | ([IW] ; loc ; [LKW])
> > >  include "cos-opt.cat"
> > >  let W = W | UL
> > >  let M = R | W
> > 
> > No idea.  But the following litmus test gets no executions whatsoever,
> > so point taken about my missing at least one corner case.  ;-)
> > 
> > Adding a spin_unlock() to the end of either process allows both to
> > run.
> > 
> > One could argue that this is a bug, but one could equally well argue
> > that if you have a deadlock, you have a deadlock.
> > 
> 
> in lock.cat: 
> 
> 	(* Allow up to one unmatched LKW per location; more must deadlock *)
> 	let UNMATCHED-LKW = LKW \ domain(critical)
> 	empty ([UNMATCHED-LKW] ; loc ; [UNMATCHED-LKW]) \ id as unmatched-locks
> 
> we rule out deadlocks from the execution candidates we care about.

Thank you, Boqun!

							Thanx, Paul

> Regards,
> Boqun
> 
> > Thoughts?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > C lock
> > 
> > {
> > }
> > 
> > 
> > P0(int *a, int *b, spinlock_t *l)
> > {
> > 	spin_lock(l);
> > 	WRITE_ONCE(*a, 1);
> > }
> > 
> > P1(int *a, int *b, spinlock_t *l)
> > {
> > 	spin_lock(l);
> > 	WRITE_ONCE(*b, 1);
> > }
> > 
> > exists (a=1 /\ b=1)