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SubjectRe: Some -serious- BPF-related litmus tests
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@fb.com/
> > >
> > > 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

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