Re: [PATCH RFC tools/memory-model] Add s390.{cfg,cat}

Paul E. McKenney
2018-04-02T19:31:54Z

On Thu, Mar 29, 2018 at 10:40:43AM -0400, Alan Stern wrote:
> On Wed, 28 Mar 2018, Paul E. McKenney wrote:
>
> > > > In the meantime, does the cat file look to you like it correctly
> > > > models the combination of TSO and multicopy atomicity? Do the
> > > > fences really work, or did I just get lucky with my choice of
> > > > litmus tests?
> > >
> > > You got lucky. Try creating an SB litmus test where, instead of an
> > > smp_mb() fence between the write and the read, each thread executes
> > > some other kind of fence.
> >
> > Ah, it does indeed get "Never" in that case, which I do not believe
> > to e correct.
> >
> > > The acyclicity condition should have been written more like this:
> > >
> > > let po_ghb = ([R] ; po ; [M]) | ([M] ; po ; [W])
> > >
> > > acyclic mfence | po_ghb | rf | fr | co as tso-mca
> > >
> > > I don't know what the fence instruction is on s390; change the "mfence"
> > > above accordingly. The main difference between this and the
> > > corresponding expression in x86tso.cat is that I replaced rfe with rf.
> >
> > The s390 fence instruction is "bcr 14,0" or "bcr 15,0", depending on
> > how recent of hardware you are running. The latter works everywhere,
> > if I recall correctly. But I do not believe that herd knows about either
> > instruction yet.
>
> Herd does not need to understand s390 assembly in order to handle the
> things defined in linux.def, such as "smp_mb()". linux.def doesn't
> contain any x86 assembly language stuff either (or PPC or ARM).
>
> > Ah, and I need to lose the "empty rmw & (fre;coe)".
> > That appears to be where my spurious ordering was coming from, strange
> > though that seems to me.
>
> No, don't drop it; it was not the source of your spurious ordering.
> The extra ordering came from your "(po \ (W * R))" term, which
> unintentionally matches fences as well as memory accesses.
>
> > And your use of "rf" instead of "rfe" makes sense, as that is what makes
> > the read-from-write provide ordering, correct? And that should also cover
> > the "Uniproc check" that would otherwise be required, right?
>
> I don't think so...
>
> > Except that I get "Sometimes" on CoWR+poonceonce+Once.litmus...
>
> Exactly.
>
> > Which I can fix by unioning po-loc into po-ghb. Or is there some
> > better way to do this?
>
> You could just keep the "uniproc" check. These two approaches accept
> the same set of litmus tests.
>
> Logically, I think of these as two distinct categories of ordering.
> po_ghb and tso-mca have to do with the order in which stores reach the
> cache, whereas "uniproc" (AKA sequential consistency per variable) has
> to do with enforcement of the cache coherence requirements. Clearly
> they are related, but they aren't the same thing.
>
> > > This doesn't account for atomic operations properly; see the "implied"
> > > term in x86tso.cat.
> >
> > I will look at this more later, reaching end of both battery and useful
> > attention span...

Like the following, perhaps?

Thanx, Paul

------------------------------------------------------------------------

s390

include "fences.cat"
include "cos.cat"

(* Fundamental coherence ordering *)
let com = rf | co | fr
acyclic po-loc | com as coherence

(* Atomic *)
empty rmw & (fre;coe) as atom

(* Fences *)
let mb = [M] ; fencerel(Mb) ; [M]

(* TSO with multicopy atomicity *)
let po-ghb = ([R] ; po ; [M]) | ([M] ; po ; [W])
acyclic mb | po-ghb | fr | rf | co as sc