linux-kernel - Re: Question about DEC Alpha memory ordering

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Date:   Wed, 15 Feb 2017 08:26:46 +1300
From:   Michael Cree <mcree@...on.net.nz>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     klausman@...warzvogel.de,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        stern@...land.harvard.edu, bob smith <sfmc68@...izon.net>,
        rth@...ddle.net, ink@...assic.park.msu.ru, mattst88@...il.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, will.deacon@....com,
        linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Question about DEC Alpha memory ordering

On Tue, Feb 14, 2017 at 12:35:58PM +0100, Andrea Parri wrote:
> On Mon, Feb 13, 2017 at 01:24:36PM -0800, Paul E. McKenney wrote:
> > ------------------------------------------------------------------------
> > 
> > C auto/C-LB-LRW+OB-Ov
> > (*
> >  * Result: Maybe
> >  * P0-P1 rf OB-Ov: Never->Maybe: Note lack of C11 guarantee, control dependency
> >  * P1 Ov,LRW: Note lack of C11 guarantee, control dependency
> >  *)
> > {
> > }
> > 
> > P0(int *u0, int *x1)
> > {
> > 	r1 = READ_ONCE(*u0);
> > 	smp_mb();
> > 	WRITE_ONCE(*x1, 1);
> > }
> > 
> > 
> > P1(int *u0, int *x1)
> > {
> > 	r1 = READ_ONCE(*x1);
> > 	WRITE_ONCE(*u0, r1);
> > }
> > 
> > exists
> > (0:r1=1 /\ 1:r1=1)
> > 
> 
> The (automatically generated) module for this test is at
> 
>    http://retis.sssup.it/~a.parri/lkmm/C-LB-LRW+OB-Ov.tgz ;
> 
> the test is run by cat-ing /sys/kernel/litmus/p_count: this will execute
> the thread bodies for "runs * size" iterations; results can be sentisive
> to the "stride" and "affinity increment" parameters (c.f., the Makefile);
> statistics for each experiments are printed on stdout.

This is the test run on a 3-cpu ES45 with the settings in the Makefile:

Test auto/LB-LRW+OB-Ov Allowed
Histogram (2 states)
5913093	:> 0:r1=0; 1:r1=0; 
4086907	:> 0:r1=0; 1:r1=1; 
No
Witnesses
Positive: 0 Negative: 10000000
Condition exists (0:r1=1 /\ 1:r1=1) is NOT validated
Observation auto/LB-LRW+OB-Ov Never 0 10000000
Time auto/LB-LRW+OB-Ov 9.570
Hash=200258693ffc841829310726a4a0b7e3

How do we interpret these results?

Cheers
Michael.