Messages in this thread Patch in this message | | | Date | Fri, 20 Feb 2009 12:09:29 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH URCU formal] Add liveness checks to user-level RCU model. |
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On Fri, Feb 20, 2009 at 02:46:21PM -0500, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > On Fri, Feb 20, 2009 at 01:18:35PM -0500, Mathieu Desnoyers wrote: > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > Break all potentially infinite loops in both urcu_reader() and > > > > urcu_updater(), ensure that urcu_reader() will process any memory barriers > > > > that urcu_updater() might issue, and formulate a "never" claim that checks > > > > to make sure that if either urcu_reader() or urcu_updater() completes, > > > > then the other will eventually also complete. Since urcu_reader() > > > > now has a finite number of steps, it must eventually complete. > > > > > > > > Also replace the code at the end of urcu_reader() that previously absorbed > > > > late memory-barrier requests from urcu_updater with code in urcu_writer() > > > > that checks to see if urcu_reader() has completed. > > > > > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > > > Thanks Paul, I'll merge it. However, I am currently reworking our spin > > > tree so we can execute the tests in batch (rather that all at once, > > > which consumes more memory than necessary) and also I am doing a nice > > > build script which lets us create our own LTL formulaes for > > > verification. The never claims will be automatically generated and > > > verified. I'll keep you posted. > > > > Sounds interesting! Not sure what you mean by "execute the tests > > in batch", but look forward to seeing it. > > > > On the LTL, the formula "<>[] (reader_done != 0 && updater_done != 0)" > > didn't do what I want. The model would kick out an error with the > > reader sitting just before the "reader_done = 1" and the updater spinning > > waiting for the reader to respond to its memory-barrier request. > > > > So I fell back to the hand-coded formula in the never clause, which > > translates to English as "if either the reader or the updater complete, > > then both the reader and the updater eventually complete". There might > > be a way to tranlate that into LTL, but I didn't immediately see one. > > > > This morning I tried the weak fairness constraints (the "-f" argument > > to ./pan) and that did allow LTL to do what I want, as shown in the > > following patch (applied on top of my earlier patch). > > > > I must confess that LTL is at best an acquired taste for me. > > "Let's see... '<>[](!reader_done || !updater_done)'... > > That means eventually we always must have neither the reader or the > > updater being done. Huh??? Oh, yeah, this is supposed to say what > > -cannot- happen..." At this point, I have an easier time with the > > hand-coded "never" claims. ;-) > > > > But I am quite happy to leave further hacking on this model in > > your capable hands. The other item on my todo list was making the > > urcu_mbmin.spin model accurately handle omission of additional memory > > barriers. Are you willing to take that on as well? > > I'll first get the translation of asserts into LTL formulaes, and try to > see what should be fixed in the model. I have noticed that we would need > to do this : > > urcu_gp_ctr = (urcu_gp_ctr + RCU_GP_CTR_BIT) % (RCU_GP_CTR_BIT + 1); > > Otherwise the overflow does not do what we expect (spin -f on the trail > told me that it was overflowing to 1, which is not exactly what we want > I guess). More to come on that side. When this will be settled, I'll dig > further.
Hmmm... The two legal values for urcu_gp_ctr are 1 and 129, so isn't this in fact the desired behavior? This is with your optimization that cuts a half-cycle from rcu_read_lock() by making the initial value of urcu_gp_ctr be 1 rather than 0.
One way of getting rid of the warning would be something like the following:
atomic { if :: urcu_gp_ctr == 1 -> urcu_gp_ctr = 129; :: urcu_gp_ctr == 129 -> urcu_gp_ctr = 1; fi; }
On converting the assert() to LTL, better you than me! :-)
Thanx, Paul
> Mathieu > > > > Thanx, Paul > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > --- > > > > urcu.sh | 4 ++-- > > urcu.spin | 12 ------------ > > 2 files changed, 2 insertions(+), 14 deletions(-) > > > > diff --git a/formal-model/urcu.sh b/formal-model/urcu.sh > > index 5e525ec..3a6850c 100644 > > --- a/formal-model/urcu.sh > > +++ b/formal-model/urcu.sh > > @@ -20,6 +20,6 @@ > > # > > # Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > -spin -a urcu.spin > > +spin -a -f '<>[](!reader_done || !updater_done)' urcu.spin > > cc -o pan pan.c > > -./pan -a > > +./pan -a -f > > diff --git a/formal-model/urcu.spin b/formal-model/urcu.spin > > index cf1f670..851eb50 100644 > > --- a/formal-model/urcu.spin > > +++ b/formal-model/urcu.spin > > @@ -280,15 +280,3 @@ init { > > run urcu_updater(); > > } > > } > > - > > -/* Require that both reader and updater eventually get done. */ > > - > > -never { > > - do > > - :: skip; > > - :: reader_done != 0 || updater_done != 0 -> break; > > - od; > > -accept: do > > - :: reader_done == 0 || updater_done == 0; > > - od; > > -} > > > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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