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Date:   Fri, 22 May 2020 10:45:40 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Peter Zijlstra <peterz@infradead.org>, parri.andrea@gmail.com,
        will@kernel.org, boqun.feng@gmail.com, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        andriin@fb.com
Subject: Re: Some -serious- BPF-related litmus tests
Message-ID: <20200522174540.GK2869@paulmck-ThinkPad-P72>
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On Fri, May 22, 2020 at 10:36:09AM -0400, Alan Stern wrote:
> 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.

I suspect that you are right.  I asked him if there were other accesses
in my response to his ringbuffer (as opposed to litmus-test) patch:

https://lore.kernel.org/bpf/20200522002502.GF2869@paulmck-ThinkPad-P72/

If there are other accesses requiring both, the litmus tests might need
to be updated.

							Thanx, Paul