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SubjectRe: Checkin 7404ad3b6d04efbd918e9e2e776bf560fbedf47d breaks boot on KVM
James Bottomley wrote:
>>
>> :040000 040000 98fc7ae95211b8d16e2e8ca46997be660ad9ba93
>> 2d35d0a2b6232411b840a9ccf6a52b946172566e M drivers
>>
>> However, simply reverting this commit caused a panic on boot (not
>> entirely surprising.)
>
> Actually, it is surprising. That patch takes the default arithmetic for
> calculating the disk size out of sd and uses a routine to do it more
> efficiently in lib/string_helpers.c
>
> So there are two problems: Why does it panic on revert (could you post
> the oops) and what does kvm object to in string_get_size ... it's a
> fairly innocuous routine as I read it ... your symptoms sound like the
> for loop isn't terminating.
>

Looks like it's trying to print a zero. I made a diff to print out the
raw numbers, and did indeed get zero.

Now, *why* it is printing a zero is another matter (as is why using
ffz() instead of ilog2() there...). Some quick investigation showed
that sdkp->capacity is a 32-bit quantity in this configuration, and
shifting it left by 9 of course ends up with zero; being a virtual disk,
it's an exact power of two.

Still, it is bad that string_get_size() hangs on passing zero.

The spinlock lockup is still happening, so I'm assuming it is an
unrelated bug that was masked by the string_get_size issue.

Patch series will follow shortly.

-hpa


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