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

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Date:	Fri, 29 Jun 2012 16:01:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970

On Sat, Jun 30, 2012 at 12:40:07AM +0200, Sasha Levin wrote:
> 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.

Very good -- if an alleged fix survives for 10 minutes, we have excellent
statistical confidence that it is in fact a fix.

							Thanx, Paul

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