linux-kernel - Re: crash in 3.12.51 (likely in 3.12.52 as well) in timer code

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Message-ID: <56B34233.5010804@kyup.com>
Date:	Thu, 4 Feb 2016 14:21:07 +0200
From:	Nikolay Borisov <kernel@...p.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Cc:	Jiri Slaby <jslaby@...e.cz>, Oleg Nesterov <oleg@...hat.com>,
	tglx@...utronix.de,
	SiteGround Operations <operations@...eground.com>
Subject: Re: crash in 3.12.51 (likely in 3.12.52 as well) in timer code



On 02/04/2016 02:17 PM, Mike Galbraith wrote:
> On Thu, 2016-02-04 at 13:51 +0200, Nikolay Borisov wrote:
>>
>> On 02/04/2016 01:32 PM, Mike Galbraith wrote:
>>> On Wed, 2016-02-03 at 12:58 +0200, Nikolay Borisov wrote:
>>>>
>>>> So in this case the prev/next entries do not look like corrupted,
>>>> whereas
>>>> when manipulating the list inside detach_timer they do. This is
>>>> really
>>>> odd, any ideas how to further debug this?
>>>
>>> Suspiciously similar to https://lkml.org/lkml/2016/2/4/247
>>
>> Right, I've been cursory following this thread but I was left with the
>> impression this only occurs on machines where the CPU can go offline,
>> currently the server on which this happened should never offline any of
>> its CPUs since the power management is disabled (though I will have to
>> double check this).
> 
> AFAIU, hotplug isn't required, only mod_delayed_work() being called
> from a different CPU than where the timer was born, migrating it at a
> bad time.

Right, in this case the ib_addr was indeed using mod_delayed_work so
things line up so far.

> 
>> On a different note - is there a way to safely reproduce this so I can
>> test the suggested fix by Thomas?
> 
> Hm, write a module to beat mod_delayed_work() to pulp with a NR_CPUS
> horde, and run it in a vm where you don't care about shrapnel?

In other words, have multiple threads (NR_CPUS) that spin on
mod_delayed_work?


> 
> 	-Mike
>