linux-kernel - Re: Some -serious- BPF-related litmus tests

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Message-ID: <20200522105659.GH2869@paulmck-ThinkPad-P72>
Date:   Fri, 22 May 2020 03:56:59 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     stern@...land.harvard.edu, parri.andrea@...il.com, will@...nel.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        andriin@...com
Subject: Re: Some -serious- BPF-related litmus tests

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

							Thanx, Paul