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Re: [libvirt] [PATCH 1/1] qemu: Tidy up job handling during live migration



On 30/07/14 01:52, Ján Tomko wrote:
> On 07/29/2014 02:41 AM, Sam Bobroff wrote:
>> During a QEMU live migration several warning messages about job
>> handling could be written to syslog on the destination host:
>>
>> "entering monitor without asking for a nested job is dangerous"
>>
>> The messages are written because the job handling during migration
>> uses hard coded asyncJob values in several places that are incorrect.
>>
>> This patch passes the required asyncJob value around and prevents
>> the warnings as well as any issues that the warnings may be referring
>> to.
>>
>> Signed-off-by: Sam Bobroff <sam bobroff au1 ibm com>
>> --
> 
> This patch seems to fix the deadlock that can happen if the migrated domain is
> destroyed at the destination reported here:
> https://www.redhat.com/archives/libvir-list/2014-May/msg00236.html
> 
> It looks good to me, but it seems there are more functions calling
> qemuDomainObjEnterMonitor that can be called from qemuProcessStart,
> for example qemuDomainChangeGraphicsPasswords.

Yes, I was fairly sure there would be other cases; I just fixed all the
ones that actually occurred during my tests.

In fact it seems like for the cases I'm looking at here, where it's the
async job owner thread that's using the EnterMonitor functions, passing
asyncJob around is a waste of time anyway because we know the correct
value of asyncJob to use: it's stored in priv->job.asyncJob.

Why not have qemuDomainObjEnterMonitorInternal() automatically switch to
creating a nested job in this case?

It seems easy to do and would simplify some code as well; what do you think?

>> @@ -2505,7 +2506,7 @@ qemuDomainUpdateDeviceList(virQEMUDriverPtr driver,
>>      if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_DEVICE_DEL_EVENT))
>>          return 0;
>>  
>> -    qemuDomainObjEnterMonitor(driver, vm);
>> +    ignore_value(qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob));
> 
> Also, the return value of this call could be dangerous to ignore if asyncJob
> != NONE.

True, but the patch hasn't introduced this, and the full story is even
worse ;-)

 void qemuDomainObjEnterMonitor(virQEMUDriverPtr driver,
                               virDomainObjPtr obj)
 {
    ignore_value(qemuDomainObjEnterMonitorInternal(driver, obj,
                                                   QEMU_ASYNC_JOB_NONE));
 }

> 
> Jan

Cheers,
Sam.


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