Feedback-ID: iad51458e:Fastmail
Date:   Sun, 29 Jan 2023 13:43:53 -0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
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
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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.

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)