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

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(a)redhat.com&gt; >>>>>> >>>>>> 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(a)redhat.com&gt; >>>>>> --- >>>>>> 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@entry=0x7fffd4211090, config=0x0, json=false, >>>>> cb=cb@entry=0x7fffddcae720 <monitorCallbacks>, >>>>> opaque=opaque@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.