Re: [PATCH 2/2 v2 RFC] tools/memory-model: redefine rb in terms of rcu-fence

[Boqun Feng]

On Thu, Mar 01, 2018 at 10:37:58AM -0800, Paul E. McKenney wrote:
> On Thu, Mar 01, 2018 at 09:49:06AM -0800, Paul E. McKenney wrote:
> > On Thu, Mar 01, 2018 at 10:49:05AM -0500, Alan Stern wrote:
> > > On Thu, 1 Mar 2018, Boqun Feng wrote:
> > > 
> > > > > +let rec rcu-fence = gp |
> > > > > +	(gp ; rcu-link ; rscs) |
> > > > > +	(rscs ; rcu-link ; gp) |
> > > > > +	(gp ; rcu-link ; rcu-fence ; rcu-link ; rscs) |
> > > > > +	(rscs ; rcu-link ; rcu-fence ; rcu-link ; gp) |
> > > > > +	(rcu-fence ; rcu-link ; rcu-fence)
> > > > > +
> > > > > +(* rb orders instructions just as pb does *)
> > > > > +let rb = prop ; rcu-fence ; hb* ; pb*
> > > > >  
> > > > >  irreflexive rb as rcu
> > > > 
> > > > I wonder whether we can simplify things as:
> > > > 
> > > > 	let rec rcu-fence =
> > > > 	    (gp; rcu-link; rscs) |
> > > > 	    (rscs; rcu-link; gp) |
> > > > 	    (gp; rcu-link; rcu-fence; rcu-link; rscs) |
> > > > 	    (rscs; rcu-link; rcu-fence; rcu-link; gp)
> > > > 	
> > > > 	(* gp and rcu-fence; rcu-link; rcu-fence removed *)
> > > > 	
> > > > 	let rb = prop; rcu-fence; hb*; pb*
> > > > 
> > > > 	acycle rb as rcu
> > > > 
> > > > In this way, "rcu-fence" is defined as "any sequence containing as many
> > > > grace periods as RCU read-side critical sections (joined by rcu-link)."
> > > > Note that "rcu-link" contains "gp", so we don't miss the case where
> > > > there are more grace periods. And since we use "acycle" now, so we don't
> > > > need "rcu-fence; rcu-link; rcu-fence" to build "rcu-fence" recursively.
> > > 
> > > Would this definition of rcu-fence work for a sequence such as (leaving
> > > out the intermediate rcu-link parts):
> > > 
> > > 	gp gp gp rscs rscs gp rscs rscs
> > > 
> > > ?  I don't think it would.  Yes, if you had a cycle of that form then 

Right. 

> > > your "rcu" axiom would detect it, but at some point we might want to 
> > > use rcu-fence for some other purpose, one that doesn't involve cycles.

OK, and I've not yet found another simple way to express rcu-fence for
purposes other than cycle-checking. So I'm OK to leave it as it is
except removing the redundant "gp" in rcu-fence definition.

But I will continue to search for a easier and sufficient way to define
these things ;-) 

> > 
> > Let's see, that would map to this:
> > 
> > auto/RW-G+RW-G+RW-G+RW-R+RW-R+RW-G+RW-R+RW-R.litmus
> > 
> > And no, there is no such automatically generated litmus test.  Let's
> > try reversing the "gp" and "rscs", which should have the same effect
> > courtesy of symmetry:
> > 
> > auto/RW-R+RW-R+RW-R+RW-G+RW-G+RW-R+RW-G+RW-G.litmus
> > 
> > And that one doesn't exist, either.  So much for random test generation!  :-/
> > 
> > Clearly time to add them.  And here is what herd has to say about them:
> > 
> > l$ sh scripts/checklitmus.sh /tmp/auto/C-RW-G+RW-G+RW-G+RW-R+RW-R+RW-G+RW-R+RW-R.litmus
> > Herd options: -conf linux-kernel.cfg
> > Observation /tmp/auto/C-RW-G+RW-G+RW-G+RW-R+RW-R+RW-G+RW-R+RW-R Sometimes 1 255
> >  ^^^ Unexpected non-Never verification
> >  0inputs+32outputs (0major+2605minor)pagefaults 0swaps
> > $ sh scripts/checklitmus.sh /tmp/auto/C-RW-R+RW-R+RW-R+RW-G+RW-G+RW-R+RW-G+RW-G.litmus
> > Herd options: -conf linux-kernel.cfg
> > Observation /tmp/auto/C-RW-R+RW-R+RW-R+RW-G+RW-G+RW-R+RW-G+RW-G Sometimes 1 255
> >  ^^^ Unexpected non-Never verification
> >  0inputs+32outputs (0major+2620minor)pagefaults 0swaps
> > 
> > In other words, they are in fact misclassified as "Sometimes" when they
> > should be "Never".  I have my diffs below in case I misapplied Boqun's
> > change.
> > 
> > With Alan's original formulation, these two litmus tests are correctly
> > handled:
> > 
> > $ sh scripts/checklitmus.sh /tmp/auto/C-RW-G+RW-G+RW-G+RW-R+RW-R+RW-G+RW-R+RW-R.litmus
> > Herd options: -conf linux-kernel.cfg
> > Observation /tmp/auto/C-RW-G+RW-G+RW-G+RW-R+RW-R+RW-G+RW-R+RW-R Never 0 255
> > 1.61user 0.00system 0:01.63elapsed 98%CPU (0avgtext+0avgdata 9572maxresident)k
> > $ sh scripts/checklitmus.sh /tmp/auto/C-RW-R+RW-R+RW-R+RW-G+RW-G+RW-R+RW-G+RW-G.litmus
> > Herd options: -conf linux-kernel.cfg
> > Observation /tmp/auto/C-RW-R+RW-R+RW-R+RW-G+RW-G+RW-R+RW-G+RW-G Never 0 255
> > 1.84user 0.01system 0:01.92elapsed 96%CPU (0avgtext+0avgdata 10112maxresident)k
> 
> And as Andrea pointed out off-list, I did indeed mess up Boqun's change.
> I forgot to change the "irreflexive" into "acyclic".  Applying that change
> makes everything work.
> 
> Please accept my apologies for my confusion!
> 

np, also I should have provided a proper patch for your testing.

For this Alan's patch, feel free to add:

Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>

Regards,
Boqun

> 							Thanx, Paul
> 
> > > > I prefer this because we already treat "gp" as "strong-fence", which
> > > > already is a "rcu-link".
> > > 
> > > That's a good point; it had not occurred to me.
> > 
> > And if I remove the "gp" but leave the last line, it does properly
> > classify the two new litmus tests.
> > 
> > 							Thanx, Paul
> > 
> > > >  Also, recurisively extending rcu-fence with
> > > > itself is exactly calculating the transitive closure, which we can avoid
> > > > by using a "acycle" rule. Besides, it looks more consistent with hb and
> > > > pb.
> > > 
> > > That _had_ occurred to me.  But I couldn't see any way to do it while 
> > > still defining rcu-fence correctly.
> > 
> > ------------------------------------------------------------------------
> > 
> > diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> > index 1e5c4653dd12..75d3c225146c 100644
> > --- a/tools/memory-model/linux-kernel.cat
> > +++ b/tools/memory-model/linux-kernel.cat
> > @@ -106,12 +106,11 @@ let rcu-link = hb* ; pb* ; prop
> >   * Any sequence containing at least as many grace periods as RCU read-side
> >   * critical sections (joined by rcu-link) acts as a generalized strong fence.
> >   *)
> > -let rec rcu-fence = gp |
> > +let rec rcu-fence =
> >  	(gp ; rcu-link ; rscs) |
> >  	(rscs ; rcu-link ; gp) |
> >  	(gp ; rcu-link ; rcu-fence ; rcu-link ; rscs) |
> > -	(rscs ; rcu-link ; rcu-fence ; rcu-link ; gp) |
> > -	(rcu-fence ; rcu-link ; rcu-fence)
> > +	(rscs ; rcu-link ; rcu-fence ; rcu-link ; gp)
> >  
> >  (* rb orders instructions just as pb does *)
> >  let rb = prop ; rcu-fence ; hb* ; pb*
>

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