linux-kernel - Re: BUG: tick device NULL pointer during system initialization and shutdown

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Date:	Mon, 08 Jul 2013 09:04:31 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>, athorlton@....com,
	CAI Qian <caiqian@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: BUG: tick device NULL pointer during system initialization and
 shutdown



On 07/01/2013 09:30 AM, Thomas Gleixner wrote:
> On Mon, 1 Jul 2013, Prarit Bhargava wrote:
>> On 06/28/2013 06:52 AM, Thomas Gleixner wrote:
>>> Huch. Did the warning in the broadcast code trigger before that?
>>
>> tglx,
>>
>> AFAICT it does not.  Log below on the system I'm testing on.  The test on the
>> system is system boots, sleeps for 30 seconds and then reboots.
> 
>> [  270.563197] INFO: rcu_sched detected stalls on CPUs/tasks: { 51} (detected by
>> 63, t=217205 jiffies, g=3583, c=3582, q=578)
> 
> So the stall is on CPU51, but we do not get a backtrace for CPU51. 
> 
> The backtrace trigger is only sent to online cpus. So CPU51 is offline
> already. Which makes sense as we are in the process of bringing CPUs
> down and the CPUs with backtrace are 0 and 53-63.
> 
> I'm pretty sure, that the patch which clears the stale flag is
> unrelated to this and it cures the NULL pointer dereference (the
> reason why this can happen is clear).
> 
> So now you do not longer trip over the NULL pointer dereference, but
> you see a weird RCU stall on an already DEAD cpu. Note, it's dead
> because we already took CPU52 offline as well.
> 
> Paul???

I hit this a few times ... but the frequency of hitting this is MUCH less than
that off the original bug.  So Thomas, can you add

Tested-by: Prarit Bhargava <prarit@...hat.com>

to the "tick: Make oneshot broadcast robust vs. CPU offlining" patch?

IMO that problem seems to be solved and we're just peeling the proverbial onion
and finding deeper bugs.

P.
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