linux-kernel - Re: [PATCH] Fixed division by zero bug in kernel/padata.c

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Date:	Mon, 5 Jul 2010 17:35:32 +0400
From:	Dan Kruchinin <dkruchinin@....org>
To:	Steffen Klassert <steffen.klassert@...unet.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH] Fixed division by zero bug in kernel/padata.c

On Mon, Jul 5, 2010 at 5:18 PM, Steffen Klassert
<steffen.klassert@...unet.com> wrote:
> On Fri, Jul 02, 2010 at 05:24:13PM +0400, Dan Kruchinin wrote:
>> No problem. Here is fixed patch:
>> --
>>  When boot CPU(typically CPU #0) is excluded from padata cpumask and
>>  user enters halt command from console, kernel faults on division by zero;
>>  This occurs because during the halt kernel shuts down each non-boot CPU one
>>  by one. After it shuts down the last CPU that is set in the padata cpumask,
>>  the only working CPU in the system is a boot CPU(#0) and it's the only CPU that
>>  is set in the cpu_active_mask. Hence when padata_cpu_callback calls
>>  __padata_remove_cpu(and hence padata_alloc_pd) it appears that padata
>> cpumask and
>>  cpu_active mask aren't intersect. Hence the following code in
>> padata_alloc_pd causes
>>  a DZ error exception:
>>   cpumask_and(pd->cpumask, cpumask, cpu_active_mask); // pd->cpumask
>> will be empty
>>   ...
>>   num_cpus = cpumask_weight(pd->cpumask); // num_cpus = 0
>>   pd->max_seq_nr = (MAX_SEQ_NR / num_cpus) * num_cpus - 1; // DZ!
>>
>
> I'm still thinking about how to handle an empty cpumask here.
> While your patch would be ok to handle the shutdown case you
> noticed, the problem is a bit more complex as soon as we are
> able to change the cpumasks from userspace with your patches.
>
> Essentially, we can end up with an empty cpumask here because
> of two reasons:
>
> 1. A user removed the last cpu that belongs to the padata
> cpumask and the active cpumask.
>
> 2. The last cpu that belongs to the padata cpumask and the
> active cpumask goes offline.
>
> In the first case it would be ok to tell the user that this is
> an invalid operation by returning an error. In the second case
> we can't just return an error to the cpu hotplug callback function,
> because it returns NOTIFY_BAD on error. This means, that it depends
> on the padata user configuration whether a cpu can go offline or not.
> This is certainly not what we want to have.
>
> Both cases should be handled in the same way. So we could just
> stop the instance if the cpumasks do not intersect, and enable
> it as soon as they do intersect again. The padata instance would
> refuse to do anything as long as the cpumasks do not intersect,
> but it is still in a consistent state. Let me add the infrastructure
> to handle this, then you can use it with your patches.

Ok, get it.

>
> Thanks,
>
> Steffen
>



-- 
W.B.R.
Dan Kruchinin
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