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SubjectRe: [PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier
On Wed, Oct 14, 2015 at 11:04:19PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 14, 2015 at 01:19:17PM -0700, Paul E. McKenney wrote:
> > Suppose we have something like the following, where "a" and "x" are both
> > initially zero:
> >
> > CPU 0 CPU 1
> > ----- -----
> >
> > WRITE_ONCE(x, 1); WRITE_ONCE(a, 2);
> > r3 = xchg(&a, 1); smp_mb();
> > r3 = READ_ONCE(x);
> >
> > If xchg() is fully ordered, we should never observe both CPUs'
> > r3 values being zero, correct?
> >
> > And wouldn't this be represented by the following litmus test?
> >
> > PPC SB+lwsync-RMW2-lwsync+st-sync-leading
> > ""
> > {
> > 0:r1=1; 0:r2=x; 0:r3=3; 0:r10=0 ; 0:r11=0; 0:r12=a;
> > 1:r1=2; 1:r2=x; 1:r3=3; 1:r10=0 ; 1:r11=0; 1:r12=a;
> > }
> > P0 | P1 ;
> > stw r1,0(r2) | stw r1,0(r12) ;
> > lwsync | sync ;
> > lwarx r11,r10,r12 | lwz r3,0(r2) ;
> > stwcx. r1,r10,r12 | ;
> > bne Fail0 | ;
> > mr r3,r11 | ;
> > Fail0: | ;
> > exists
> > (0:r3=0 /\ a=2 /\ 1:r3=0)
> >
> > I left off P0's trailing sync because there is nothing for it to order
> > against in this particular litmus test. I tried adding it and verified
> > that it has no effect.
> >
> > Am I missing something here? If not, it seems to me that you need
> > the leading lwsync to instead be a sync.
>
> So the scenario that would fail would be this one, right?
>
> a = x = 0
>
> CPU0 CPU1
>
> r3 = load_locked (&a);
> a = 2;
> sync();
> r3 = x;
> x = 1;
> lwsync();
> if (!store_cond(&a, 1))
> goto again
>
>
> Where we hoist the load way up because lwsync allows this.

That scenario would end up with a==1 rather than a==2.

> I always thought this would fail because CPU1's store to @a would fail
> the store_cond() on CPU0 and we'd do the 'again' thing, re-issuing the
> load and now seeing the new value (2).

The stwcx. failure was one thing that prevented a number of other
misordering cases. The problem is that we have to let go of the notion
of an implicit global clock.

To that end, the herd tool can make a diagram of what it thought
happened, and I have attached it. I used this diagram to try and force
this scenario at https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html#PPC,
and succeeded. Here is the sequence of events:

o Commit P0's write. The model offers to propagate this write
to the coherence point and to P1, but don't do so yet.

o Commit P1's write. Similar offers, but don't take them up yet.

o Commit P0's lwsync.

o Execute P0's lwarx, which reads a=0. Then commit it.

o Commit P0's stwcx. as successful. This stores a=1.

o Commit P0's branch (not taken).

o Commit P0's final register-to-register move.

o Commit P1's sync instruction.

o There is now nothing that can happen in either processor.
P0 is done, and P1 is waiting for its sync. Therefore,
propagate P1's a=2 write to the coherence point and to
the other thread.

o There is still nothing that can happen in either processor.
So pick the barrier propagate, then the acknowledge sync.

o P1 can now execute its read from x. Because P0's write to
x is still waiting to propagate to P1, this still reads
x=0. Execute and commit, and we now have both r3 registers
equal to zero and the final value a=2.

o Clean up by propagating the write to x everywhere, and
propagating the lwsync.

And the "exists" clause really does trigger: 0:r3=0; 1:r3=0; [a]=2;

I am still not 100% confident of my litmus test. It is quite possible
that I lost something in translation, but that is looking less likely.

> > Of course, if I am not missing something, then this applies also to the
> > value-returning RMW atomic operations that you pulled this pattern from.
> > If so, it would seem that I didn't think through all the possibilities
> > back when PPC_ATOMIC_EXIT_BARRIER moved to sync... In fact, I believe
> > that I worried about the RMW atomic operation acting as a barrier,
> > but not as the load/store itself. :-/
>
> AARGH64 does something very similar; it does something like:
>
> ll
> ...
> sc-release
>
> mb
>
> Which I assumed worked for the same reason, any change to the variable
> would fail the sc, and we go for round 2, now observing the new value.

I have to defer to Will on this one. You are right that ARM and PowerPC
do have similar memory models, but there are some differences.

Thanx, Paul
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