Re: LKMM: Making RMW barriers explicit

Boqun Feng
2024-05-23T15:14:42Z

On Thu, May 23, 2024 at 04:26:23PM +0200, Hernan Ponce de Leon wrote:
> On 5/23/2024 4:05 PM, Alan Stern wrote:
> > On Thu, May 23, 2024 at 02:54:05PM +0200, Jonas Oberhauser wrote:
> > >
> > >
> > > Am 5/22/2024 um 4:20 PM schrieb Alan Stern:
> > > > It would be better if there was a way to tell herd7 not to add the 'mb
> > > > tag to failed instructions in the first place. This approach is
> > > > brittle; see below.
> > >
> > > Hernan told me that in fact that is actually currently the case in herd7.
> > > Failing RMW get assigned the Once tag implicitly.
> > > Another thing that I'd suggest to change.
> >
> > Indeed.
> >
> > > > An alternative would be to have a way for the .cat file to remove the
> > > > 'mb tag from a failed RMW instruction. But I don't know if this is
> > > > feasible.
> > >
> > > For Mb it's feasible, as there is no Mb read or Mb store.
> > >
> > > Mb = Mb & (~M | dom(rmw) | range(rmw))
> > >
> > > However one would want to do the same for Acq and Rel.
> > >
> > > For that one would need to distinguish e.g. between a read that comes from a
> > > failed rmw instruction, and where the tag would disappear, or a normal
> > > standalone read.
> > >
> > > For example, by using two different acquire tags, 'acquire and 'rmw-acquire,
> > > and defining
> > >
> > > Acquire = Acquire | Rmw-acquire & (dom(rmw) | range(rmw))
> > >
> > > Anyways we can do this change independently. So for now, we don't need
> > > RMW_MB.
> >
> > Overall, it seems better to have herd7 assign the right tag, but change
> > the way the .def file works so that it can tell herd7 which tag to use
> > in each of the success and failure cases.
>
> I am not fully sure how herd7 uses the .def file, but I guess something like
> adding a second memory tag to __cmpxchg could work
>
> cmpxchg(X,V,W) __cmpxchg{mb, once}(X,V,W)
> cmpxchg_relaxed(X,V,W) __cmpxchg{once, once}(X,V,W)
> cmpxchg_acquire(X,V,W) __cmpxchg{acquire, acquire}(X,V,W)
> cmpxchg_release(X,V,W) __cmpxchg{release, release}(X,V,W)
>

Note that cmpxchg_acquire() and cmpxchg_release() don't have _acqurie
or _release ordering if they fails.

Besides, I'm not sure this is a good idea. Because the "{mb}, {once},
etc" part is a syntax thing, you write a cmpxchg(), it should be
translated to a cmpxchg event with MB tag on. As to failed cmpxchg()
doesn't provide ordering, it's a semantics thing, as Jonas showed that
it can be represent in cat file. As long as it's a semanitc thing and we
can represent in cat file, I don't think we want herd to give a special
treatment.

What you and Jonas looks fine to me, since it moves the semantic bits
from herd internal to cat file.

Regards,
Boqun

> Hernan
>
> >
> > > > [M] ; po ; [RMW_MB]
> > > >
> > > > [RMW_MB] ; po ; [M]
> > > >
> > > > This is because events tagged with RMW_MB always are memory accesses,
> > > > and accesses that aren't part of the RMW are already covered by the
> > > > fencerel(Mb) thing above.
> > >
> > > This has exactly the issue mentioned above - it will cause the rmw to have
> > > an internal strong fence that on powerpc probably isn't there.
> >
> > Oops, that's right. Silly oversight on my part. But at least you
> > understood what I meant.
> >
> > > We could do (with the assumption that Mb applies only to successful rmw):
> > >
> > > [M] ; po ; [Mb & R]
> > > [Mb & W] ; po ; [M]
> >
> > That works.
> >
> > Alan
>