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

[Andrea Parri]

> > 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