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

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Message-ID: <20110513071850.GU2258@linux.vnet.ibm.com>
Date:	Fri, 13 May 2011 00:18:50 -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 Thu, May 12, 2011 at 12:05:24PM -0400, Valdis.Kletnieks@...edu wrote:
> On Thu, 12 May 2011 02:47:05 PDT, "Paul E. McKenney" said:
> > On Wed, May 11, 2011 at 07:11:34PM -0400, Valdis.Kletnieks@...edu wrote:
> > > 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.
> 
> Ah. Found it. Your tree and current linux-next include this commit:
> 
> commit	1217ed1ba5c67393293dfb0f03c353b118dadeb4
> tree	a765356c8418e134de85fd05d9fe6eda41de859c	tree | snapshot
> parent	29ce831000081dd757d3116bf774aafffc4b6b20	commit | diff
> rcu: permit rcu_read_unlock() to be called while holding runqueue locks
> 
> which includes this chunk:
> 
> @@ -1546,8 +1531,8 @@ static void rcu_cpu_kthread_timer(unsigned long arg)
> 
>         raw_spin_lock_irqsave(&rnp->lock, flags);
>         rnp->wakemask |= rdp->grpmask;
> -       invoke_rcu_node_kthread(rnp);
>         raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +       invoke_rcu_node_kthread(rnp);
>  }
> 
> 
> but that was committed 4 days ago, and Andrew pulled linux-next for the -mmotm
> 6 days ago, so it's not in there.  The *rest* of your recent commits appear to
> be in there though.  So that explains the patch failure to apply.

Whew!!!  ;-)

							Thanx, Paul
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