Re: Question about DEC Alpha memory ordering

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: bob smith <sfmc68@verizon.net>,
	rth@twiddle.net, ink@jurassic.park.msu.ru, mattst88@gmail.com,
	j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
	parri.andrea@gmail.com, will.deacon@arm.com,
	linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: Question about DEC Alpha memory ordering
Date: Mon, 13 Feb 2017 13:24:36 -0800	[thread overview]
Message-ID: <20170213212436.GQ30506@linux.vnet.ibm.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1702131604170.1905-100000@iolanthe.rowland.org>

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)