[libvirt] [PATCH] Fix race condition reconnecting to vms & loading configs

[Daniel P. Berrange]

On Mon, Oct 28, 2013 at 01:22:39PM -0400, Cole Robinson wrote:
> On 10/28/2013 01:14 PM, Daniel P. Berrange wrote:
> > On Mon, Oct 28, 2013 at 01:08:45PM -0400, Cole Robinson wrote:
> >> On 10/28/2013 01:06 PM, Daniel P. Berrange wrote:
> >>> On Mon, Oct 28, 2013 at 01:03:49PM -0400, Cole Robinson wrote:
> >>>> On 10/28/2013 07:52 AM, Daniel P. Berrange wrote:
> >>>>> From: "Daniel P. Berrange" <berrange at redhat.com>
> >>>>>
> >>>>> The following sequence
> >>>>>
> >>>>>  1. Define a persistent QMEU guest
> >>>>>  2. Start the QEMU guest
> >>>>>  3. Stop libvirtd
> >>>>>  4. Kill the QEMU process
> >>>>>  5. Start libvirtd
> >>>>>  6. List persistent guets
> >>>>>
> >>>>> At the last step, the previously running persistent guest
> >>>>> will be missing. This is because of a race condition in the
> >>>>> QEMU driver startup code. It does
> >>>>>
> >>>>>  1. Load all VM state files
> >>>>>  2. Spawn thread to reconnect to each VM
> >>>>>  3. Load all VM config files
> >>>>>
> >>>>> Only at the end of step 3, does the 'virDomainObjPtr' get
> >>>>> marked as "persistent". There is therefore a window where
> >>>>> the thread reconnecting to the VM will remove the persistent
> >>>>> VM from the list.
> >>>>>
> >>>>> The easy fix is to simply switch the order of steps 2 & 3.
> >>>>>
> >>>>> Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
> >>>>> ---
> >>>>>  src/qemu/qemu_driver.c | 3 +--
> >>>>>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> >>>>> index c613967..9c3daad 100644
> >>>>> --- a/src/qemu/qemu_driver.c
> >>>>> +++ b/src/qemu/qemu_driver.c
> >>>>> @@ -816,8 +816,6 @@ qemuStateInitialize(bool privileged,
> >>>>>  
> >>>>>      conn = virConnectOpen(cfg->uri);
> >>>>>  
> >>>>> -    qemuProcessReconnectAll(conn, qemu_driver);
> >>>>> -
> >>>>>      /* Then inactive persistent configs */
> >>>>>      if (virDomainObjListLoadAllConfigs(qemu_driver->domains,
> >>>>>                                         cfg->configDir,
> >>>>> @@ -828,6 +826,7 @@ qemuStateInitialize(bool privileged,
> >>>>>                                         NULL, NULL) < 0)
> >>>>>          goto error;
> >>>>>  
> >>>>> +    qemuProcessReconnectAll(conn, qemu_driver);
> >>>>>  
> >>>>>      virDomainObjListForEach(qemu_driver->domains,
> >>>>>                              qemuDomainSnapshotLoad,
> >>>>>
> >>>>
> >>>> I tried testing this patch to see if it would fix:
> >>>>
> >>>> https://bugzilla.redhat.com/show_bug.cgi?id=1015246
> >>>>
> >>>> from current master I did:
> >>>>
> >>>> git revert a924d9d083c215df6044387057c501d9aa338b96
> >>>> reproduce the bug
> >>>> git am <your-patch>
> >>>>
> >>>> But the daemon won't even start up after your patch is built:
> >>>>
> >>>> (gdb) bt
> >>>> #0  qemuMonitorOpen (vm=vm at entry=0x7fffd4211090, config=0x0, json=false,
> >>>>     cb=cb at entry=0x7fffddcae720 <monitorCallbacks>,
> >>>>     opaque=opaque at entry=0x7fffd419b840) at qemu/qemu_monitor.c:852
> > 
> >> Sorry for not being clear: The daemon crashes, that's the backtrace.
> > 
> > Hmm config is NULL - does the state XML files not include the
> > monitor info perhaps ?
> > 
> 
> I see:
> 
> pidfile for busted VM in /var/run/libvirt/qemu
> nothing in /var/cache/libvirt/qemu
> no state that I can see in /var/lib/libvirt/qemu
> 
> But I'm not sure where it's supposed to be stored.
> 
> FWIW reproducing this state was pretty simple: revert
> a924d9d083c215df6044387057c501d9aa338b96, edit an existing x86 guest to remove
> all <video> and <graphics> devices, start the guest, libvirtd crashes.

Ok, I believe you probably have SELinux disabled on your machine or in
libvirtd. With SELinux enabled you hit another bug first

2013-10-29 13:50:11.711+0000: 17579: error : qemuConnectMonitor:1401 : Failed to set security context for monitor for rhel6x86_64


which prevents hitting the crash you report. The fix is the same in both
cases - we must skip VMs with PID of zero. I've sent a v2 patch.


Daniel
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