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

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

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:
> > On Thu, Jan 14, 2016 at 02:55:10PM -0800, Paul E. McKenney wrote:
> > > On Thu, Jan 14, 2016 at 01:36:50PM -0800, Leonid Yegoshin wrote:
> > > > On 01/14/2016 01:29 PM, Paul E. McKenney wrote:
> > > > >
> > > > >>On 01/14/2016 12:34 PM, Paul E. McKenney wrote:
> > > > >>>
> > > > >>>The WRC+addr+addr is OK because data dependencies are not required 
> > > > >>>to be
> > > > >>>transitive, in other words, they are not required to flow from one 
> > > > >>>CPU to
> > > > >>>another without the help of an explicit memory barrier.
> > > > >>I don't see any reliable way to fit WRC+addr+addr into "DATA
> > > > >>DEPENDENCY BARRIERS" section recommendation to have data dependency
> > > > >>barrier between read of a shared pointer/index and read the shared
> > > > >>data based on that pointer. If you have this two reads, it doesn't
> > > > >>matter the rest of scenario, you should put the dependency barrier
> > > > >>in code anyway. If you don't do it in WRC+addr+addr scenario then
> > > > >>after years it can be easily changed to different scenario which
> > > > >>fits some of scenario in "DATA DEPENDENCY BARRIERS" section and
> > > > >>fails.
> > > > >The trick is that lockless_dereference() contains an
> > > > >smp_read_barrier_depends():
> > > > >
> > > > >#define lockless_dereference(p) \
> > > > >({ \
> > > > >       typeof(p) _________p1 = READ_ONCE(p); \
> > > > >       smp_read_barrier_depends(); /* Dependency order vs. p above. */ 
> > > > > \
> > > > >       (_________p1); \
> > > > >})
> > > > >
> > > > >Or am I missing your point?
> > > > 
> > > > WRC+addr+addr has no any barrier. lockless_dereference() has a
> > > > barrier. I don't see a common points between this and that in your
> > > > answer, sorry.
> > > 
> > > Me, I am wondering what WRC+addr+addr has to do with anything at all.
> > 
> > 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?

        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?  ;-)

                                                        Thanx, Paul

PS.  And yes, working through this does help me understand the
     benefits of fake dependencies.  Why do you ask?  ;-)

------------------------------------------------------------------------

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)


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