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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