Re: LKMM: Making RMW barriers explicit

Boqun Feng
2024-05-24T14:34:09Z

On Fri, May 24, 2024 at 10:14:25AM -0400, Alan Stern wrote:
[...]
> > Not really, RMW events contains all events generated from
> > read-modify-write primitives, if there is an R event without a rmw
> > relation (i.e there is no corresponding write event), it's a failed RWM
> > by definition: it cannot be anything else.
>
> Not true. An R event without an rmw relation could be a READ_ONCE().

No, the R event is already in the RWM events, it has come from a rwm
atomic.

> Or a plain read. The memory model uses the tag to distinguish these
> cases.
>
> > > that it would work is merely an artifact of herd7's internal actions. I
> > > think it would be much cleaner if herd7 represented these events in some
> > > other way, particularly if we can tell it how.
> > >
> > > After all, herd7 already does generate different sets of events for
> > > successful (both R and W) and failed (only R) RMWs. It's not such a big
> > > stretch to make the R events it generates different in the two cases.
> > >
> >
> > I thought we want to simplify the herd internal processing by avoid
> > adding mb events in cmpxchg() cases, in the same spirit, if syntactic
> > tagging is already good enough, why do we want to make herd complicate?
>
> Herd7 already is complicated by the fact that it needs to handle
> cmpxchg() instructions in two ways: success and failure. This
> complication is unavoidable. Adding one extra layer (different tags for
> the different ways) is an insignificant increase in the complication,
> IMO. And it's a net reduction when you compare it to the amount of
> complication currently in the herd7 code.
>
> Also what about cmpxchg_acquire()? If it fails, it will generate an R
> event with an acquire tag not in the rmw relation. There is no way to
> tell such events apart from a normal smp_load_acquire(), and so the .cat
> file would have no way to know that the event should not have acquire
> semantics. I guess we would have to rename this tag, as well.

No,

let read_of_rmw = (RMW & R)
let fail_read_of_rmw = read_of_rmw / dom(rmw)
let fail_cmpxchg_acquire = fail_read_of_rmw & [Acquire]

gives you all the failed cmpxchg_acquire() apart from a normal
smp_load_acquire().

Regards,
Boqun

>
> Alan