Re: [PATCH 2/2] powerpc/iommu: check dev->iommu_group before remove a device from iommu_group

[Wei Yang <weiyang@linux.vnet.ibm.com>]

On Thu, Aug 22, 2013 at 10:17:20AM -0600, Alex Williamson wrote:
>On Thu, 2013-08-22 at 23:41 +0800, Wei Yang wrote:
>> >> 
>> >> Alex,
>> >> 
>> >> Sorry for not including you in the very beginning, which may spend you more
>> >> efforts to track previous mails in this thread.
>> >> 
>> >> Do you think it is reasonable to check the dev->iommu_group in
>> >> iommu_group_remove_device()? Or we can count on the bus notifier to check it?
>> >> 
>> >> Welcome your suggestions~
>> >
>> >I don't really see the point of patch 1/2. iommu_group_remove_device()
>> >is specifically to remove a device from an iommu_group, so why would you
>> >call it on a device that's not part of an iommu_group.  If you want to
>> >avoid testing dev->iommu_group, then implement the .remove_device
>> >callback rather than using the notifier.  Thanks,
>> >
>> 
>> You mean the .remove_device like intel_iommu_remove_device()? 
>> 
>> Hmm... this function didn't check the dev->iommu_group and just call
>> iommu_group_remove_device(). I see this guard is put in iommu_bus_notifier(), 
>> which will check dev->iommu_group before invoke .remove_device.
>> 
>> Let me explain the case to triger the problem a little. 
>> 
>> On some platform, like powernv, we implement another bus notifier when devices
>> are added or removed in the system. Like Alexey mentioned, he missed the check
>> for dev->iommu_group in the notifier before removing it from iommu_group. This
>> trigger the crash.
>> 
>> So do you think it is reasonable to guard the kernel in
>> iommu_group_remove_device(), or we give the platform developers the
>> responsibility to check the dev->iommu_group before calling it?
>
>I don't see it as we need either patch 1/2 or patch 2/2.  We absolutely
>need some form of patch 2/2.  Patch 1/2 isn't necessarily bad, but it
>facilitates sloppy usage.  The iommu driver shouldn't be calling
>iommu_group_remove_device() on arbitrary devices that may or may not be
>part of an iommu_group.  Perhaps patch 1/2 should be:
>
>if (WARN_ON(!group))
>	return;
>

Agree, this one sounds more reasonable. :-)

Since patch 2/2 is merged by Alexey, I will re-send patch 1/2 alone.

Thanks for your comments ~

>Thanks,
>
>Alex
>
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-- 
Richard Yang
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