[cpp-threads] SC on PPC (was Re: Increment/decrement operators on atomics package)

[Alexander Terekhov]

On 4/30/07, Paul E. McKenney <paulmck at linux.vnet.ibm.com> wrote:
> On Mon, Apr 30, 2007 at 09:09:12PM +0200, Alexander Terekhov wrote:
> > On 4/30/07, Raul Silvera <rauls at ca.ibm.com> wrote:
> > >
> > >Alexander Terekhov wrote on 04/30/2007 08:10:19 AM:
> > >
> > >> How does cumulativity help in the IRIW case?
> > >>
> > >> P1: x = 1;
> > >> P2: y = 1;
> > >> P3: r1 = x; r2 = y;
> > >> P4: r3 = y; r4 = x;
> > >>
> > >
> > >The short version of this is that cumulativity on a hwsync between the two
> > >loads on P3 would cause a StoreLoad ordering between P1's store to x
> > >and P3's load of y.
> >
> > This is rather intriguing because unless I'm just missing something,
> > cumulativity is defined the same for all barriers including
> > lwsync/eieio and it comes into play when *P3* makes a post-barrier
> > store which is observed by another processor.
>
> Not for isync or for ordering based on dependencies, however.

But isync is not really a bidirectional barrier. It's just a way to
achieve .acq using fake or real dependencies.

>
> B-cumulativity does indeed cover the case where a post-barrier store
> is observed by another processor.  In this case, subsequent operations
> on this other processor will observe all applicable pre-barrier
> operations.  Of course, "subsequent" must be enforced, and in the
> case of "lwsync", a pair consisting of a pre-barrier store and a
> post-barrier load is not "applicable".
>
> A-cumulativity covers the case where a pre-barrier operation
> observes an operation by some other CPU, and forces post-barrier
> operations to also observe that other CPU's operation.

To me, cummulative "post barrier" means that we have something in the
B set (after expansion) on another processor "... performed after a
Load instruction executed by that processor or mechanism has returned
the value stored by a store that is in B."

http://www.decadentplace.org.uk/pipermail/cpp-threads/2007-January/001369.html
http://www.decadentplace.org.uk/pipermail/cpp-threads/2007-January/001373.html

IOW with an empty "B-cumulativity", whatever you add to the A set
(calling it "A-cumulativity") is not constrained at all with respect
to other processors.

regards,
alexander.