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

[Daniel P. Berrange]

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 ?

Daniel
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