On Fri, Aug 26, 2022 at 01:42:19PM -0700, Paul E. McKenney wrote:
> On Fri, Aug 26, 2022 at 01:10:39PM -0400, Alan Stern wrote:
> > On Fri, Aug 26, 2022 at 06:23:24PM +0200, Peter Zijlstra wrote:
> > > I think we should address that first one in LKMM, it seems very weird to
> > > me a RmW would break the chain like that.
> >
> > An explicitly relaxed RMW (atomic_cmpxchg_relaxed(), to be precise).
> >
> > If the authors wanted to keep the release-acquire chain intact, why not
> > use a cmpxchg version that has release semantics instead of going out of
> > their way to use a relaxed version?
> >
> > To put it another way, RMW accesses and release-acquire accesses are
> > unrelated concepts. You can have one without the other (in principle,
> > anyway). So a relaxed RMW is just as capable of breaking a
> > release-acquire chain as any other relaxed operation is.
> >
> > > Is there actual hardware that
> > > doesn't behave?
> >
> > Not as far as I know, although that isn't very far. Certainly an
> > other-multicopy-atomic architecture would make the litmus test succeed.
> > But the LKMM does not require other-multicopy-atomicity.
>
> My first attempt with ppcmem suggests that powerpc does -not- behave
> this way. But that surprises me, just on general principles. Most likely
> I blew the litmus test shown below.
>
> Thoughts?
The litmus test looks okay.
As for your surprise, remember that PPC is B-cumulative, another
property which the LKMM does not require. B-cumulativity will also
force the original litmus test to succeed. (The situation is like ISA2
in the infamous test6.pdf, except that y and z are separate variables in
ISA2 but are the same here. The RMW nature of lwarx/stwcx provides
the necessary R-W ordering in P1.)
Alan
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> PPC MP+lwsyncs+atomic
> "LwSyncdWW Rfe LwSyncdRR Fre"
> Cycle=Rfe LwSyncdRR Fre LwSyncdWW
> {
> 0:r2=x; 0:r4=y;
> 1:r2=y; 1:r5=2;
> 2:r2=y; 2:r4=x;
> }
> P0 | P1 | P2 ;
> li r1,1 | lwarx r1,r0,r2 | lwz r1,0(r2) ;
> stw r1,0(r2) | stwcx. r5,r0,r2 | lwsync ;
> lwsync | | lwz r3,0(r4) ;
> li r3,1 | | ;
> stw r3,0(r4) | | ;
> exists (1:r1=1 /\ 2:r1=2 /\ 2:r3=0)
>
> ------------------------------------------------------------------------
>
> $ ./ppcmem -model lwsync_read_block -model coherence_points MP+lwsyncs+atomic.litmus
> ...
> Test MP+lwsyncs+atomic Allowed
> States 9
> 1:r1=0; 2:r1=0; 2:r3=0;
> 1:r1=0; 2:r1=0; 2:r3=1;
> 1:r1=0; 2:r1=1; 2:r3=1;
> 1:r1=0; 2:r1=2; 2:r3=0;
> 1:r1=0; 2:r1=2; 2:r3=1;
> 1:r1=1; 2:r1=0; 2:r3=0;
> 1:r1=1; 2:r1=0; 2:r3=1;
> 1:r1=1; 2:r1=1; 2:r3=1;
> 1:r1=1; 2:r1=2; 2:r3=1;
> No (allowed not found)
> Condition exists (1:r1=1 /\ 2:r1=2 /\ 2:r3=0)
> Hash=b7cec0e2ecbd1cb68fe500d6fe362f9c
> Observation MP+lwsyncs+atomic Never 0 9
