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

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Message-ID: <Pine.LNX.4.44L0.1702131604170.1905-100000@iolanthe.rowland.org>
Date:   Mon, 13 Feb 2017 16:06:21 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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>,
        <parri.andrea@...il.com>, <will.deacon@....com>,
        <linux-alpha@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: Question about DEC Alpha memory ordering

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)

Alan