Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

[Boqun Feng <boqun.feng@xxxxxxxxx>]

Hi Will,

On Tue, Jan 26, 2016 at 12:16:09PM +0000, Will Deacon wrote:
> On Mon, Jan 25, 2016 at 10:03:22PM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 25, 2016 at 04:42:43PM +0000, Will Deacon wrote:
> > > On Fri, Jan 15, 2016 at 01:58:53PM -0800, Paul E. McKenney wrote:
> > > > PPC Overlapping Group-B sets version 4
> > > > ""
> > > > (* When the Group-B sets from two different barriers involve 
> > > > instructions in
> > > >    the same thread, within that thread one set must contain the other.
> > > > 
> > > >         P0      P1      P2
> > > >         Rx=1    Wy=1    Wz=2
> > > >         dep.    lwsync  lwsync
> > > >         Ry=0    Wz=1    Wx=1
> > > >         Rz=1
> > > > 
> > > >         assert(!(z=2))
> > > > 
> > > >    Forbidden by ppcmem, allowed by herd.
> > > > *)
> > > > {
> > > > 0:r1=x; 0:r2=y; 0:r3=z;
> > > > 1:r1=x; 1:r2=y; 1:r3=z; 1:r4=1;
> > > > 2:r1=x; 2:r2=y; 2:r3=z; 2:r4=1; 2:r5=2;
> > > > }
> > > >  P0             | P1            | P2            ;
> > > >  lwz r6,0(r1)   | stw r4,0(r2)  | stw r5,0(r3)  ;
> > > >  xor r7,r6,r6   | lwsync        | lwsync        ;
> > > >  lwzx r7,r7,r2  | stw r4,0(r3)  | stw r4,0(r1)  ;
> > > >  lwz r8,0(r3)   |               |               ;
> > > > 
> > > > exists
> > > > (z=2 /\ 0:r6=1 /\ 0:r7=0 /\ 0:r8=1)
> > > 
> > > That really hurts. Assuming that the "assert(!(z=2))" is actually there
> > > to constrain the coherence order of z to be {0->1->2}, then I think that
> > > this test is forbidden on arm using dmb instead of lwsync. That said, I
> > > also don't think the Rz=1 in P0 changes anything.
> > 
> > What about the smp_wmb() variant of dmb that orders only stores?
> 
> Tricky, but I think it still works out if the coherence order of z is as
> I described above. The line of reasoning is weird though -- I ended up
> considering the two cases where P0 reads z before and after it reads x
                                             ^^^^^^^^^^^^^^^
Because of the fact that two reads on the same processors can't be
executed simultaneously? I feel like this is exactly something herd
missed.

> and what that means for the read of y.
> 

And the reasoning on PPC is similar, so looks like the read of z on P0
is a necessary condition for the exists clause to be forbidden.

Regards,
Boqun

> Will

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