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

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Message-ID: <20200522174352.GJ2869@paulmck-ThinkPad-P72>
Date:   Fri, 22 May 2020 10:43:52 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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 10:32:01AM -0400, Alan Stern 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, it looks like one or the other of those is redundant (depending 
> on the context).

Probably.  Peter instead asked what it was supposed to even do.  ;-)

> Also, what use is a spinlock that is accessed in only one thread?

Multiple writers synchronize via the spinlock in this case.  I am
guessing that his larger 16-hour test contended this spinlock.

> Finally, I doubt that these tests belong under tools/memory-model.  
> Shouldn't they go under the new Documentation/ directory for litmus 
> tests?  And shouldn't the patch update a README file?

Agreed, and I responded to that effect to his original patch:

https://lore.kernel.org/bpf/20200522003433.GG2869@paulmck-ThinkPad-P72/

							Thanx, Paul