Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

[Jonas Oberhauser]

On 1/25/2023 9:36 PM, Andrea Parri wrote:
> > > Why do you want the implementation to forbid it?  The pattern of the
> > > litmus test resembles 3+3W, and you don't care whether the kernel allows
> > > that pattern.  Do you?
> > Jonas asked a similar question, so I am answering you both here.
> >
> > With (say) a release-WRITE_ONCE() chain implementing N+2W for some
> > N, it is reasonably well known that you don't get ordering, hardware
> > support otwithstanding.  After all, none of the Linux kernel, C, and C++
> > memory models make that guarantee.  In addition, the non-RCU barriers
> > and accesses that you can use to create N+2W have been in very wide use
> > for a very long time.
> >
> > Although RCU has been in use for almost as long as those non-RCU barriers,
> > it has not been in wide use for anywhere near that long.  So I cannot
> > be so confident in ruling out some N+2W use case for RCU.
> Did some archeology...  the pattern, with either RCU sync plus a release
> or with two full fences plus a release, was forbidden by "ancient LKMM":
> the relevant changes were described in
>
>    https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/LWNLinuxMM/WeakModel.html#Coherence%20Point%20and%20RCU
>
>    Andrea

Fascinating! It says there "But the weak model allows it, as required" 
-- what does "as required" mean? Just "as required by dropping the 
constraint"?

Is there still a notion of "strong model" and "weak model", or was the 
strong model dropped?

jonas