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

[Cole Robinson]

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
>> #1  0x00007fffdda1083d in qemuConnectMonitor (
>>     driver=driver at entry=0x7fffd419b840, vm=vm at entry=0x7fffd4211090,
>>     logfd=logfd at entry=-1) at qemu/qemu_process.c:1412
>> #2  0x00007fffdda1685a in qemuProcessReconnect (
>>     opaque=opaque at entry=0x7fffd422fef0) at qemu/qemu_process.c:3086
>> #3  0x00007ffff7528dce in virThreadHelper (data=<optimized out>)
>>     at util/virthreadpthread.c:161
>> #4  0x00007ffff4782f33 in start_thread (arg=0x7fffcb7fe700)
>>     at pthread_create.c:309
>> #5  0x00007ffff40a9ead in clone ()
>>     at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111
> 
> What is this trace showing ? or rather what is the error reported
> when it fails to start ?
> 

Sorry for not being clear: The daemon crashes, that's the backtrace.

- Cole