Re: Some -serious- BPF-related litmus tests

[Alan Stern]

On Fri, May 22, 2020 at 03:56:59AM -0700, Paul E. McKenney wrote:
> 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 wasn't the context in the litmus test.  The context was:

	smp_wmb();
	smp_store_release();
	spin_unlock();
	smp_store_release();

That certainly looks like a lot more ordering than is really needed.

Alan