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

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Date:   Mon, 16 Jan 2023 14:13:57 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "parri.andrea" <parri.andrea@...il.com>, 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 Mon, Jan 16, 2023 at 02:20:57PM -0500, Alan Stern wrote:
> On Mon, Jan 16, 2023 at 11:06:52AM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 16, 2023 at 01:11:41PM -0500, Alan Stern wrote:
> 
> > > Why do you want to prohibit nesting?  Why would that be a better 
> > > approximation?
> > 
> > Because the current LKMM gives wrong answers for nested critical
> > sections.
> 
> I don't agree.  Or at least, it depends on whose definition of "nested 
> critical sections" you adopt.

Fair point, and I have therefore updated the test's header comment
to read as follows:

(*
 * Result: Sometimes
 *
 * This demonstrates non-nested overlapping of SRCU read-side critical
 * sections.  Unlike RCU, SRCU critical sections do not unconditionally
 * nest.
 *)

> >  For example, for the litmus test shown below, mainline
> > LKMM will incorrectly report "Never".  The two SRCU read-side critical
> > sections are independent, so the fact that P1()'s synchronize_srcu() is
> > guaranteed to wait for the first on to complete says nothing about the
> > second having completed.  Therefore, in Linux-kernel SRCU, the "exists"
> > clause could be satisfied.
> > 
> > In contrast, the proposed change flags this as having nesting.
> 
> In fact, this litmus test has overlapping critical sections, not nested 
> ones.  But the current LKML incorrectly _thinks_ they are nested, 
> because it matches each lock with the first unmatched unlock.
> 
> If you write a litmus test that has properly nested (not overlapping!) 
> read-side critical sections, the current LKMM will match the locks and 
> unlocks correctly and will give the right answer.
> 
> So what you really want to do is rule out overlapping, not nesting.  But 
> I guess there's no way to do one without the other.

None that I could see!

							Thanx, Paul

> Alan
> 
> > 							Thaxn, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > C C-srcu-nest-5
> > 
> > (*
> >  * Result: Sometimes
> >  *
> >  * This demonstrates non-nesting of SRCU read-side critical sections.
> >  * Unlike RCU, SRCU critical sections do not nest.
> >  *)
> > 
> > {}
> > 
> > P0(int *x, int *y, struct srcu_struct *s1)
> > {
> > 	int r1;
> > 	int r2;
> > 	int r3;
> > 	int r4;
> > 
> > 	r3 = srcu_read_lock(s1);
> > 	r2 = READ_ONCE(*y);
> > 	r4 = srcu_read_lock(s1);
> > 	srcu_read_unlock(s1, r3);
> > 	r1 = READ_ONCE(*x);
> > 	srcu_read_unlock(s1, r4);
> > }
> > 
> > P1(int *x, int *y, struct srcu_struct *s1)
> > {
> > 	WRITE_ONCE(*y, 1);
> > 	synchronize_srcu(s1);
> > 	WRITE_ONCE(*x, 1);
> > }
> > 
> > locations [0:r1]
> > exists (0:r1=1 /\ 0:r2=0)