Date:   Sun, 29 Jan 2023 20:43:05 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Boqun Feng <boqun.feng@gmail.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        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: <20230130044305.GO2948950@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
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On Sun, Jan 29, 2023 at 09:18:24PM -0500, Alan Stern wrote:
> On Sun, Jan 29, 2023 at 03:09:00PM -0800, Paul E. McKenney wrote:
> > On Sun, Jan 29, 2023 at 01:43:53PM -0800, Boqun Feng wrote:
> > > 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!
> 
> Actually that's only part of it.  The other part is rather obscure:
> 
> (* Generate all co relations, including LKW events but not UL *)
> let co0 = co0 | ([IW] ; loc ; [LKW]) |
> 	(([LKW] ; loc ; [UNMATCHED-LKW]) \ [UNMATCHED-LKW])
> 
> Implicitly this says that any lock with no corresponding unlock must 
> come last in the coherence order, which implies the unmatched-locks rule 
> (since only one lock event can be last).  By itself, the unmatched-locks 
> rule would not prevent P3 from executing before P1, provided P1 executes 
> both its lock and unlock.

And thank you, Alan, as well!

And RCU looks to operate in a similar manner:

------------------------------------------------------------------------

C rcudeadlock

{
}


P0(int *a, int *b)
{
	rcu_read_lock();
	WRITE_ONCE(*a, 1);
	synchronize_rcu();
	WRITE_ONCE(*b, 1);
	rcu_read_unlock();
}

P1(int *a, int *b)
{
	int r1;
	int r2;

	r1 = READ_ONCE(*b);
	smp_mb();
	r2 = READ_ONCE(*a);
}

exists (1:r1=1 /\ 1:r2=0)

------------------------------------------------------------------------

$ herd7 -conf linux-kernel.cfg rcudeadlock.litmus 
Test rcudeadlock Allowed
States 0
No
Witnesses
Positive: 0 Negative: 0
Condition exists (1:r1=1 /\ 1:r2=0)
Observation rcudeadlock Never 0 0
Time rcudeadlock 0.00
Hash=4f7f336ad39d724d93b089133b00d1e2

------------------------------------------------------------------------

So good enough!  ;-)

							Thanx, Paul