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Message-ID: <20230130044305.GO2948950@paulmck-ThinkPad-P17-Gen-1>
Date: Sun, 29 Jan 2023 20:43:05 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Boqun Feng <boqun.feng@...il.com>,
Andrea Parri <parri.andrea@...il.com>,
Jonas Oberhauser <jonas.oberhauser@...weicloud.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
Subject: Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po
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
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