[libvirt] Syslog "nested job is dangerous" message

[Eric Blake]

On 07/16/2014 07:52 PM, Sam Bobroff wrote:
> Hello everyone,

[Can you configure your mailer to wrap long lines?]

> 
> After performing a migration, the syslog often contains several messages like this:
> 
> "This thread seems to be the async job owner; entering monitor without asking for a nested job is dangerous"

The sign of a bug that we need to fix.  What version of libvirt are you
using?  We may have already fixed it.

> 
> The message makes it sound as if the user has done something dangerous but, after looking at the code that produces the message, it appears to be more to do with how the libvirt job code is used.

Rather, libvirt itself has done something dangerous.

> 
> The commit that added the warning (6eede368bc8e3df2c94c2ec1a913885ce08e99db) doesn't explain why it might be dangerous...

Jiri can probably explain better, but it means there is a race condition
where libvirt can lose track of a long-running job and cause memory
corruption in its management of the guest.

> Would it be appropriate to re-word the message, or perhaps change it to a debug message so that it's not normally seen by users?

No.

> 
> Is it important to track down the cases that are generating the warning and fix them? (Could it cause some kind of significant problem?)

Yes.

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

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