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SubjectRe: Question about DEC Alpha memory ordering
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)

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