Re: LKMM: Making RMW barriers explicit

Alan Stern
2024-05-18T00:31:36Z

On Thu, May 16, 2024 at 10:44:05AM +0200, Hernan Ponce de Leon wrote:
> On 5/16/2024 10:31 AM, Jonas Oberhauser wrote:
> >
> >
> > Am 5/16/2024 um 3:43 AM schrieb Alan Stern:
> > > Hernan and Jonas:
> > >
> > > Can you explain more fully the changes you want to make to herd7 and/or
> > > the LKMM?  The goal is to make the memory barriers currently implicit in
> > > RMW operations explicit, but I couldn't understand how you propose to do
> > > this.
> > >
> > > Are you going to change herd7 somehow, and if so, how?  It seems like
> > > you should want to provide sufficient information so that the .bell
> > > and .cat files can implement the appropriate memory barriers associated
> > > with each RMW operation.  What additional information is needed?  And
> > > how (explained in English, not by quoting source code) will the .bell
> > > and .cat files make use of this information?
> > >
> > > Alan
> >
> >
> > I don't know whether herd7 needs to be changed. Probably, herd7 does the
> > following:
> > - if a tag called Mb appears on an rmw instruction (by instruction I
> > mean things like xchg(), atomic_inc_return_relaxed()), replace it with
> > one of those things:
> >   * full mb ; once (the rmw) ; full mb, if a value returning
> > (successful) rmw
> >   * once (the rmw)   otherwise
> > - everything else gets translated 1:1 into some internal representation
>
> This is my understanding from reading the source code of CSem.ml in herd7's
> repo.
>
> Also, this is exactly what dartagnan is currently doing.
>
> >
> > What I'm proposing is:
> > 1. remove this transpilation step,
> > 2. and instead allow the Mb tag to actually appear on RMW instructions
> > 3. change the cat file to explicitly define the behavior of the Mb tag
> > on RMW instructions
>
> These are the exact 3 things I changed in dartagnan for testing what Jonas
> proposed.
>
> I am not sure if further changes are needed for herd7.

Okay, good. This answers the first part of what I asked. What about
the second part? That is, how will the changes to the .def, .bell, and
.cat files achieve your goals?

Alan