Messages in this thread | | | Date | Mon, 5 Jul 2010 17:35:32 +0400 | Subject | Re: [PATCH] Fixed division by zero bug in kernel/padata.c | From | Dan Kruchinin <> |
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On Mon, Jul 5, 2010 at 5:18 PM, Steffen Klassert <steffen.klassert@secunet.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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