Re: "Verifying and Optimizing Compact NUMA-Aware Locks on Weak Memory Models"

[Paul E. McKenney]

On Sat, Aug 27, 2022 at 12:00:15PM -0400, Alan Stern wrote:
> 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.)

Got it, thank you!

							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