Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

[Andrea Parri]

On Wed, Feb 20, 2019 at 10:26:04AM +0100, Peter Zijlstra wrote:
> On Tue, Feb 19, 2019 at 06:01:17PM -0800, Paul E. McKenney wrote:
> > On Tue, Feb 19, 2019 at 11:57:37PM +0100, Andrea Parri wrote:
> > > Remove this subtle (and, AFAICT, unused) ordering: we can add it back,
> > > if necessary, but let us not encourage people to rely on this thing.
> > > 
> > > For example, the following "exists" clause can be satisfied with this
> > > change:
> > > 
> > > C dep-rfi
> > > 
> > > { }
> > > 
> > > P0(int *x, int *y)
> > > {
> > > 	WRITE_ONCE(*x, 1);
> > > 	smp_store_release(y, 1);
> > > }
> > > 
> > > P1(int *x, int *y, int *z)
> > > {
> > > 	int r0;
> > > 	int r1;
> > > 	int r2;
> > > 
> > > 	r0 = READ_ONCE(*y);
> > > 	WRITE_ONCE(*z, r0);
> > > 	r1 = smp_load_acquire(z);
> > > 	r2 = READ_ONCE(*x);
> > > }
> > > 
> > > exists (1:r0=1 /\ 1:r2=0)
> > 
> > Any objections?  If I don't hear any in a couple days, I will apply this.
> 
> IIUC you cannot build hardware that allows the above, so why would we
> allow it?

The change/simplification was mainly intended as precautionary measure
(hence the "we can add it back, ..."): I do agree that it shouldn't be
possible to realize the above state; OTOH, you really don't need to be
too "creative" to imagine possible mis-uses/mis-interpretations of the
(dep ; rfi) term ("forget" ONCEs, trick herd7 with "false dependencies"
or simply wrongly assume that control dependencies are part this "dep",
what else? ...).  So, no, I'm not that fond to this term; why should I
be?  or you are simply suggesting to expand the changelog?

I should probably also remark that "such a simplification" wouldn't be
a first time for the LKMM and, in fact, for the ppo term itself:

  https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/LWNLinuxMM/WeakModel.html#Preserved%20Program%20Order

  Andrea