Re: [PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier

[Boqun Feng]

On Thu, Oct 15, 2015 at 09:30:40AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 15, 2015 at 12:48:03PM +0800, Boqun Feng wrote:
> > On Wed, Oct 14, 2015 at 08:07:05PM -0700, Paul E. McKenney wrote:
[snip]
> > 
> > > Why not try creating a longer litmus test that requires P0's write to
> > > "a" to propagate to P1 before both processes complete?
> > > 
> > 
> > I will try to write one, but to be clear, you mean we still observe 
> > 
> > 0:r3 == 0 && a == 2 && 1:r3 == 0 
> > 
> > at the end, right? Because I understand that if P1's write to 'a'
> > doesn't override P0's, P0's write to 'a' will propagate.
> 
> Your choice.  My question is whether you can come up with a similar
> litmus test where lwsync is allowing the behavior here, but clearly
> is affecting some other aspect of ordering.
> 

Got it, though my question about the propagation of P0's write to 'a'
was originally aimed at understanding the hardware behavior(or model) in
your sequence of events ;-)

To be clear, by "some other aspect of ordering", you mean something like
a paired RELEASE+ACQUIRE senario(i.e. P1 observes P0's write to 'a' via
a load, which means P0's write to 'a' propagates at some point), right?

If so I haven't yet came up with one, and I think there's probably none,
so my worry about "lwsync" in other places is likely unnecessary.

Regards,
Boqun

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