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

On Thu, Apr 22, 2010 at 07:09, Daniel P. Berrange <berrange(a)redhat.com&gt; 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.