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

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Date:   Tue, 14 Feb 2017 12:35:58 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     klausman@...warzvogel.de,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        stern@...land.harvard.edu
Cc:     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 Mon, Feb 13, 2017 at 01:24:36PM -0800, Paul E. McKenney wrote:
> On Mon, Feb 13, 2017 at 04:06:21PM -0500, Alan Stern wrote:
> > On Mon, 13 Feb 2017, Paul E. McKenney wrote:
> > 
> > > On Mon, Feb 13, 2017 at 08:14:23PM +0100, Tobias Klausmann wrote:
> > > > Hi! 
> > > > 
> > > > On Mon, 13 Feb 2017, Paul E. McKenney wrote:
> > > > > On Mon, Feb 13, 2017 at 01:53:27PM -0500, bob smith wrote:
> > > > > > On 2/13/17 1:39 PM, Paul E. McKenney wrote:
> > > > > > > can real DEC Alpha hardware end up with both instances of "r1"
> > > > > > > having the value 1?
> > > > > > 
> > > > > > I thought this question reminded me of something, so I found this:
> > > > > > > https://www.kernel.org/doc/Documentation/memory-barriers.txt
> > > > > > 
> > > > > > and I pasted in the content - David Howells is one of the authors and
> > > > > > maybe that is why the question sort of reminded me.
> > > > > > 
> > > > > > Maybe someone has an update but this is what was said then.
> > > > > 
> > > > > Well, thank you for pointing me to this, but my question was intended to
> > > > > check whether or not the words I helped to write in memory-barriers.txt
> > > > > are in fact accurate.  So if you have an SMP DEC Alpha system that you
> > > > > could provide remote access to, that would be very helpful!
> > > > 
> > > > I have a 4-cpu ES40. Send me a test program and I'll gladly run
> > > > it for you.
> > > 
> > > Andrea, could you please convert the litmus test below and send it to
> > > Tobias?
> > > 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > C auto/C-LB-LRW+OB-Dv
> > > (*
> > >  * Result: Never
> > >  * 
> > >  *)
> > > {
> > > }
> > > 
> > > P0(int *u0, int *x1)
> > > {
> > > 	r1 = READ_ONCE(*u0);
> > > 	smp_mb();
> > > 	WRITE_ONCE(*x1, 1);
> > > }
> > > 
> > > 
> > > P1(int *u0, int *x1)
> > > {
> > > 	r1 = rcu_dereference(*x1);
> > 
> > No, please, not this.  It should be:
> > 
> > 	r1 = READ_ONCE(*x1);
> > 
> > That is, the auto/C-LB-LRW+OB-Ov.litmus test.
> > 
> > > 	WRITE_ONCE(*u0, r1);
> > > }
> > > 
> > > exists
> > > (0:r1=1 /\ 1:r1=1)
> 
> Sorry, here is the correct one in full.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> 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.

Please let me know should you find any problem with this. Thank you,

  Andrea

Disclaimer: I'm not "excited", to use an euphemism, to post such an ugly
C code to LKML ...; _most importantly_, I've certainly never tested this
on any Alpha machine ...