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Re: [libvirt] seg fault when running snapshot-create



On Thu, Apr 22, 2010 at 07:09, Daniel P. Berrange <berrange redhat com> wrote:
> On Thu, Apr 22, 2010 at 08:54:30AM -0400, Chris Lalancette wrote:
>> On 04/22/2010 08:34 AM, Daniel P. Berrange wrote:
>> > On Wed, Apr 21, 2010 at 05:16:21PM -0400, Chris Lalancette wrote:
>> >> On 04/21/2010 04:34 PM, Stephen Shaw wrote:
>> >>> I'm getting a seg fault when running virsh snapshot-create 1, but only
>> >>> when virt-manager is open and connected.
>> >>>
>> >>> Here is some of the debug info I was able to come up with -
>> >>> http://fpaste.org/9GO6/ (bt)
>> >>> http://fpaste.org/7gkH/ ('thread apply all bt)
>> >>>
>> >>> * After the crash
>> >>> (gdb) p mon->msg
>> >>> $1 = (qemuMonitorMessagePtr) 0x0
>> >>>
>> >>>
>> >>> nibbler:~ # libvirtd --version
>> >>> libvirtd (libvirt) 0.8.0
>> >>>
>> >>>
>> >>> Please let me know if there is any other information you need.
>> >>> Stephen
>> >>
>> >> Thanks for the report.  To be perfectly honest, I can't see how what
>> >> happened could happen :).  But I'll take a closer look at it and see
>> >> if I can reproduce and see what is going on with it.
>> >
>> > I see thread locking problems in the code
>> >
>> >  - qemuDomainSnapshotCreateXML() is calling monitor commands, but has
>> >    not run  qemuDomainObjBeginJobWithDriver() to ensure exclusive
>> >    access to the monitor
>> >
>> >  - qemuDomainSnapshotDiscard has same problem
>>
>> Yep, just fixing those now.  I didn't quite understand the ObjBeginJob
>> before, but I think I'm understanding it now.  This is probably the source of
>> the problems.
>
> There's some notes about the rules in src/qemu/THREADS.txt.
>
> You must acquire locks on objects in the following order, not missing
> any steps.
>
>   qemud_driver             (qemudDriverLock)
>   virDomainObjPtr          (implicit via virDomainObjFindByXXX)
>   qemuMonitorPrivatePtr    (qemuDomainObjBeginJob)
>   qemuMonitorPtr           (implicit via qemuDomainObjEnterMonitor)
>
>
> Note that qemuDomainObjEnterMonitor() will release the locks on the
> qemud_driver & virDomainObjPtr objects, once it has acquired the lock
> on qemuMonitorPtr. The qemuMonitorPrivatePtr object has a condition
> variable that ensures continued mutual exclusion, even though the
> qemud_driver, virDomainObjPtr object are now unlocked.
>
> So by missing qemuDomainObjBeginJob(), the condition variable was not
> acquired, and mutual exclusion was not assured
>
> Regards,
> Daniel

Thanks for taking care of this.  As soon as the fix is checked in I'll
rebuild libvirt and continue testing it.

Thanks,
Stephen


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