Re: [Qemu-block] [PATCH] qcow2: always initialize specific image info

[Eric Blake]

On 12/21/2015 04:05 PM, Max Reitz wrote:

>> The situation is even worse than I have feared.
>>

> Thanks for finding this!
> 
> Well, if qcow2_get_specific_info() is the only place that can actually
> cause issues in that case (i.e. calling some QMP function which uses the
> qcow2 image while the incoming_migration coroutine yields before the
> image has been fully reopened), I think the simplest way to fix this
> would be to just return NULL from qcow2_get_specific_info() in the else
> branch (which currently aborts), adding a comment how we can end up there.
> 
> However, it seems hard to believe this is the only problematic path...
> If the coroutine can yield between the BDRVQcow2State getting memset()
> to 0 and qcow2_open() having initialized it again, then any QMP command
> which makes use of the qcow2 image should fail (not necessarily
> gracefully) at that point.

I wonder if Kevin's patch will help us pinpoint culprits:

https://lists.gnu.org/archive/html/qemu-devel/2015-12/msg04096.html

> 
> I feel like qcow2_invalidate_cache() is the root cause. Basically
> completely reopening the qcow2 image just feels wrong. Maybe we could at
> least atomically replace the BDRVQcow2State, i.e. call qcow2_open()
> first (but with a new BDRVQcow2State), then swap out the BDRVQcow2State
> in the BDS, and then call qcow2_close() on the old BDRVQcow2State. That
> doesn't feel right still, but it should fix this issue, at least.
> 
> Max
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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