linux-kernel - Re: 2.6.39-rc6-mmotm0506 - lockdep splat in RCU code on page fault

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Date:	Thu, 12 May 2011 02:47:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Valdis.Kletnieks@...edu
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: 2.6.39-rc6-mmotm0506 - lockdep splat in RCU code on page fault

On Wed, May 11, 2011 at 07:11:34PM -0400, Valdis.Kletnieks@...edu wrote:
> On Tue, 10 May 2011 01:20:29 PDT, "Paul E. McKenney" said:
> 
> Would test, but it doesn't apply cleanly to my -mmotm0506 tree:
> 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 5616b17..20c22c5 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1525,13 +1525,15 @@ static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
> >   */
> >  static void rcu_cpu_kthread_timer(unsigned long arg)
> >  {
> > -	unsigned long flags;
> > +	unsigned long old;
> > +	unsigned long new;
> >  	struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, arg);
> >  	struct rcu_node *rnp = rdp->mynode;
> >  
> > -	raw_spin_lock_irqsave(&rnp->lock, flags);
> > -	rnp->wakemask |= rdp->grpmask;
> > -	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > +	do {
> > +		old = rnp->wakemask;
> > +		new = old | rdp->grpmask;
> > +	} while (cmpxchg(&rnp->wakemask, old, new) != old);
> >  	invoke_rcu_node_kthread(rnp);
> >  }
> 
> My source has this:
> 
>         raw_spin_lock_irqsave(&rnp->lock, flags);
>         rnp->wakemask |= rdp->grpmask;
>         invoke_rcu_node_kthread(rnp);
>         raw_spin_unlock_irqrestore(&rnp->lock, flags);
> 
> the last 2 lines swapped from what you diffed against.  I can easily work around
> that, except it's unclear what the implications of the invoke_rcu moving outside
> of the irq save/restore pair (or if it being inside is the actual root cause)...

Odd...

This looks to me like a recent -next -- I do not believe that straight
mmotm has rcu_cpu_kthread_timer() in it.  The patch should apply to the
last few days' -next kernels.

							Thanx, Paul
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