[libvirt] [PATCH 2/3] qemu: event: Don't fiddle with disk backing trees without a job

[Michal Privoznik]

On 08.04.2015 08:35, Michael Chapman wrote:
> On Fri, 13 Mar 2015, Peter Krempa wrote:
>> Surprisingly we did not grab a VM job when a block job finished and we'd
>> happily rewrite the backing chain data. This made it possible to crash
>> libvirt when queueing two backing chains tightly and other badness.
>>
>> To fix it, add yet another handler to the helper thread that handles
>> monitor events that require a job.
> 
> Hi Peter,
> 
> Unfortunately, I think this change has broken disk mirroring during
> migration. When a disk mirror is ready (or aborted),
> processBlockJobEvent blocks when beginning its MODIFY job because of the
> ongoing MIGRATION_OUT async job.
> 

Yes, I've already told Peter in person a week ago.

> I'd like to solve this by making use of blockJobSync from within
> qemuMigrationDriveMirror. This is also going to fix another bug where
> libvirt doesn't wait for QEMU to complete the drive mirroring before
> migrating a VM (leading to disk corruption if there's ongoing writes!)
> 
> The plan would be to move qemuBlockJobEventProcess into a separate
> qemu_blockjob.c, so it can be used from both qemu_driver and
> qemu_migration, along with a few helper functions to set up blockJobSync
> and to wait for it to be notified. Do you think this would be a suitable
> approach?
> 

Sounds good to me.

Michal