[cpp-threads] Alternatives to SC

[Raul Silvera]

cpp-threads-bounces at decadentplace.org.uk wrote on 01/16/2007 02:58:20 PM:

> On 1/16/07, Raul Silvera <rauls at ca.ibm.com> wrote:
> >
> > Alexander Terekhov" <alexander.terekhov at gmail.com:
> > >
> > > But given that for "[s]tores to storage that is Memory Coherence
> > > Required and is neither Write Through Required nor Caching Inhibited"
> > > eieio is also cumulative...
> > >
> > > P1: x = 1;
> > > P2: if (x == 1) eieio(), y = 2;
> > > P3: if (y == 2) isync(), assert(x == 1);
> > >
> > > Would also do it, I suppose. ("Storage that is Write Through Required
> > > or Caching Inhibited is not intended to be used for general-purpose
> > > programming.")
> >
> > Almost, but not quite. While it is true that the control-flow
dependence
> > orders x==1 and y=2 on P2, that ordering is not cumulative.
> > eieio does provide cumulative ordering, but only orders stores (in this
> > scenario), so it doesn't apply to the pair x==1 and y=2.
>
> Uhmm. But what we need to order is *store* to x (by P1) and *store* to
> y (by P2) both with respect to P3 (so that store to y by P2 doesn't
> become visible to P3 before store to x by P1 becomes visible to P3).
>
> Looking only locally on P2 and eieio, "A" is empty/whatever and "B"
> includes P2's store y=2. But then we apply "cumulative" expansion for
> "A" and "B" we arrive at the same result as in the case with {lw}sync
> and visibility of P1's store x=1 on P3.

You're correct. I'm sorry, my thinking before was that because A is empty
A-cumulativity wouldn't apply, but that's not correct. That is only true
for B cumulativity.

>
> If you disagree then please elaborate. (And also explain what does
> "cumulative" means for eieio and storage that is Memory Coherence
> Required and is neither Write Through Required nor Caching Inhibited).
> TIA.
>
> regards,
> alexander.
>
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