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

From: Will Deacon <[email-masked]>
Date: Mon, 25 Jan 2016 14:41:34 +0000

On Fri, Jan 15, 2016 at 11:28:45AM -0800, Paul E. McKenney wrote:
> On Fri, Jan 15, 2016 at 09:54:01AM -0800, Paul E. McKenney wrote:
> > On Fri, Jan 15, 2016 at 10:24:32AM +0000, Will Deacon wrote:
> > > See my earlier reply [1] (but also, your WRC Linux example looks more
> > > like a variant on WWC and I couldn't really follow it).
> > 
> > I will revisit my WRC Linux example.  And yes, creating litmus tests
> > that use non-fake dependencies is still a bit of an undertaking.  :-/
> > I am sure that it will seem more natural with time and experience...
> 
> Hmmm...  You are quite right, I did do WWC.  I need to change cpu2()'s
> last access from a store to a load to get WRC.  Plus the levels of
> indirection definitely didn't match up, did they?

Nope, it was pretty baffling!

> 	struct foo {
> 		struct foo *next;
> 	};
> 	struct foo a;
> 	struct foo b;
> 	struct foo c = { &a };
> 	struct foo d = { &b };
> 	struct foo x = { &c };
> 	struct foo y = { &d };
> 	struct foo *r1, *r2, *r3;
> 
> 	void cpu0(void)
> 	{
> 		WRITE_ONCE(x.next, &y);
> 	}
> 
> 	void cpu1(void)
> 	{
> 		r1 = lockless_dereference(x.next);
> 		WRITE_ONCE(r1->next, &x);
> 	}
> 
> 	void cpu2(void)
> 	{
> 		r2 = lockless_dereference(y.next);
> 		r3 = READ_ONCE(r2->next);
> 	}
> 
> In this case, it is legal to end the run with:
> 
> 	r1 == &y && r2 == &x && r3 == &c
> 
> Please see below for a ppcmem litmus test.
> 
> So, did I get it right this time?  ;-)

The code above looks correct to me (in that it matches WRC+addrs),
but your litmus test:

> PPC WRCnf+addrs
> ""
> {
> 0:r2=x; 0:r3=y;
> 1:r2=x; 1:r3=y;
> 2:r2=x; 2:r3=y;
> c=a; d=b; x=c; y=d;
> }
>  P0           | P1            | P2            ;
>  stw r3,0(r2) | lwz r8,0(r2)  | lwz r8,0(r3)  ;
>               | stw r2,0(r3)  | lwz r9,0(r8)  ;
> exists
> (1:r8=y /\ 2:r8=x /\ 2:r9=c)

Seems to be missing the address dependency on P1.

Will