Re: Some -serious- BPF-related litmus tests

[Paul E. McKenney]

On Fri, May 22, 2020 at 11:44:07AM +0200, Peter Zijlstra wrote:
> On Thu, May 21, 2020 at 05:38:50PM -0700, Paul E. McKenney wrote:
> > Hello!
> > 
> > Just wanted to call your attention to some pretty cool and pretty serious
> > litmus tests that Andrii did as part of his BPF ring-buffer work:
> > 
> > https://lore.kernel.org/bpf/20200517195727.279322-3-andriin@xxxxxx/
> > 
> > Thoughts?
> 
> I find:
> 
> 	smp_wmb()
> 	smp_store_release()
> 
> a _very_ weird construct. What is that supposed to even do?

Indeed, and I asked about that in my review of the patch containing the
code.  It -could- make sense if there is a prior read and a later store:

	r1 = READ_ONCE(a);
	WRITE_ONCE(b, 1);
	smp_wmb();
	smp_store_release(&c, 1);
	WRITE_ONCE(d, 1);

So a->c and b->c is smp_store_release() and b->d is smp_wmb().  But if
there were only stores, the smp_wmb() would suffice.  And if there wasn't
the trailing store, smp_store_release() would suffice.

But that would at least want a comment, in my opinion.  ;-)

							Thanx, Paul