netdev - Re: Soft-Lockup/Race in networking in 2.6.31-rc1+195 ( possibly?caused by netem)

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Date:	Thu, 9 Jul 2009 16:15:28 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jarek Poplawski <jarkao2@...il.com>
cc:	Andres Freund <andres@...razel.de>,
	Joao Correia <joaomiguelcorreia@...il.com>,
	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Patrick McHardy <kaber@...sh.net>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Soft-Lockup/Race in networking in 2.6.31-rc1+195 ( possibly?caused
 by netem)

On Thu, 9 Jul 2009, Jarek Poplawski wrote:
> On Thu, Jul 09, 2009 at 02:03:50PM +0200, Thomas Gleixner wrote:
> > On Thu, 9 Jul 2009, Jarek Poplawski wrote:
> > > > 
> > > > I have the feeling that the code relies on some implicit cpu
> > > > boundness, which is not longer guaranteed with the timer migration
> > > > changes, but that's a question for the network experts.
> > > 
> > > As a matter of fact, I've just looked at this __netif_schedule(),
> > > which really is cpu bound, so you might be 100% right.
> > 
> > So the watchdog is the one which causes the trouble. The patch below
> > should fix this.
> 
> I hope so. On the other hand it seems it should work with this
> migration yet, so it probably needs additional debugging.

Right. I just provided the patch to narrow down the problem, but
please test the fix of the hrtimer migration code which I sent out a
bit earlier: http://lkml.org/lkml/2009/7/9/150

It fixes a possible endless loop in the timer code which is related to
the migration changes. Looking at the backtraces of the spinlock
lockup I think that is what you hit.

       spin_lock(root_lock);
       qdisc_run(q);
	 __qdisc_run(q);
	   dequeue_skb(q);
             q->dequeue(q);
               qdisc_watchdog_schedule();
                 hrtimer_start();
                   switch_hrtimer_base(); <- loops forever

Now the other CPU is stuck in dev_xmit() spin_lock(root_lock)

Thanks,

	tglx
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