linux-kernel - Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

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Date:   Tue, 24 Jan 2023 06:54:23 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

On Tue, Jan 24, 2023 at 12:09:48PM +0100, Andrea Parri wrote:
> > There is the one below, but I am (1) not sure that I have it right,
> > (2) not immediately certain that the Linux-kernel implementation would
> > forbid it, (3) not immediately sure that it should be forbidden.
> > 
> > In the meantime, thoughts?
> 
> As it stands, P0 to completion, then P1 to completion, then P2 to
> completion should meet the "exists" clause; I guess we want "x=1"
> in the clause (or the values of the stores to "x" exchanged).

OK, so I still don't have it right.  ;-)

Make that x=1.  I think.

							Thanx, Paul

>   Andrea
> 
> 
> > ------------------------------------------------------------------------
> > 
> > C C-srcu-observed-3
> > 
> > (*
> >  * Result: Sometimes
> >  *)
> > 
> > {}
> > 
> > P0(int *x, int *y, int *z, struct srcu_struct *s)
> > {
> > 	int r1;
> > 
> > 	r1 = srcu_read_lock(s);
> > 	WRITE_ONCE(*y, 1);
> > 	WRITE_ONCE(*x, 1);
> > 	srcu_read_unlock(s, r1);
> > }
> > 
> > P1(int *x, int *y, int *z, struct srcu_struct *s)
> > {
> > 	int r1;
> > 
> > 	r1 = READ_ONCE(*y);
> > 	synchronize_srcu(s);
> > 	WRITE_ONCE(*z, 1);
> > }
> > 
> > P2(int *x, int *y, int *z, struct srcu_struct *s)
> > {
> > 	WRITE_ONCE(*z, 2);
> > 	smp_mb();
> > 	WRITE_ONCE(*x, 2);
> > }
> > 
> > exists (1:r1=1 /\ x=2 /\ z=2)