Re: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970

[Sasha Levin <levinsasha928@gmail.com>]

On Fri, 2012-06-29 at 15:27 -0700, Paul E. McKenney wrote:
> On Fri, Jun 29, 2012 at 11:40:40PM +0200, Sasha Levin wrote:
> > On Fri, 2012-06-29 at 10:23 -0700, Paul E. McKenney wrote:
> > > On Fri, Jun 29, 2012 at 12:09:44PM +0200, Sasha Levin wrote:
> > > > Hi Paul,
> > > > 
> > > > I think I've stumbled on another bug that will increase your paranoia levels even further.
> > > > 
> > > > I got the following lockup when fuzzing with trinity inside a KVM tools guest, using latest linux-next.
> > > > 
> > > > It appears that it was caused by a03d6178 ("rcu: Move RCU grace-period cleanup into kthread"). This issue doesn't reproduce easily though, it took some fuzzing before hitting it.
> > > 
> > > Hmmm...  If the preemption at that point in __rcu_read_unlock() is
> > > required to make this happen, then it would be pretty hard to hit.
> > > I suspect that you can make it reproduce more quickly by putting
> > > a udelay(10) or similar right after the assignment of INT_MIN to
> > > t->rcu_read_lock_nesting in __rcu_read_unlock() in kernel/rcupdate.c.
> > > Can this be reproduced while running with lockdep enabled?
> > 
> > The good news are that it is much easier to reproduce it by adding a udelay(10) at the point you've mentioned.
> 
> How quickly does it reproduce?

10 seconds more or less.