Messages in this thread Patch in this message | | | Date | Thu, 1 Mar 2018 09:49:06 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 2/2 v2 RFC] tools/memory-model: redefine rb in terms of rcu-fence |
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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 > 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.
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
> > 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.
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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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