On Wed, Oct 21, 2015 at 09:36:44PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 21, 2015 at 12:29:23PM -0700, Paul E. McKenney wrote:
> > On Wed, Oct 21, 2015 at 10:24:52AM +0200, Peter Zijlstra wrote:
> > > On Tue, Oct 20, 2015 at 04:34:51PM -0700, Paul E. McKenney wrote:
> > > > There is also the question of whether the barrier forces ordering
> > > > of unrelated stores, everything initially zero and all accesses
> > > > READ_ONCE() or WRITE_ONCE():
> > > >
> > > > P0 P1 P2 P3
> > > > X = 1; Y = 1; r1 = X; r3 = Y;
> > > > some_barrier(); some_barrier();
> > > > r2 = Y; r4 = X;
> > > >
> > > > P2's and P3's ordering could be globally visible without requiring
> > > > P0's and P1's independent stores to be ordered, for example, if you
> > > > used smp_rmb() for some_barrier(). In contrast, if we used smp_mb()
> > > > for barrier, everyone would agree on the order of P0's and P0's stores.
> > >
> > > Oh!?
> >
> > Behold sequential consistency, worshipped fervently by a surprisingly
> > large number of people! Something about legacy proof methods, as near
> > as I can tell. ;-)
>
> But how can smp_mb() guarantee anything about P[01]? There is but the
> single store, which can race against P[23] arbitrarily. There is nothing
> to order.
Indeed, if your barrier is only acting locally, there is no way that
you can order the two stores. However, some barriers act non-locally,
and this non-local action can order the stores. This non-locality is
"cumulativity" in the PowerPC docs.
And x86 can also order P0's and P1's stores, courtesy of the "T" in "TSO".
> Maybe I'm confused again..
They say that confusion is the most productive frame of mind...
Thanx, Paul
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