Re: [libvirt] [PATCHv2 1/2] qemu: Add missing lock of virDomainObj before calling virDomainUnref

(Eric pointed out in IRC that I should be acquiring the domainobj lock prior to modifying obj->privateData->mon) This was found while researching the root cause of: https://bugzilla.redhat.com/show_bug.cgi?id=670848 virDomainUnref should only be called with the lock held for the virDomainObj in question. However, when a transient qemu domain gets EOF on its monitor socket, it queues an event which frees the monitor, which unref's the virDomainObj without first locking it. If another thread has already locked the virDomainObj, the modification of the refcount could potentially be corrupted. In an extreme case, it could also be potentially unlocked by virDomainObjFree, thus left open to modification by anyone else who would have otherwise waited for the lock (not to mention the fact that they would be accessing freed data!). The solution is to have qemuMonitorFree lock the domain object right before unrefing it. Since the caller to qemuMonitorFree doesn't expect this lock to be held, if the refcount doesn't go all the way to 0, qemuMonitorFree must unlock it after the unref. --- src/qemu/qemu_process.c | 8 ++++++-- 1 files changed, 6 insertions(+), 2 deletions(-) diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c index c419c75..9084725 100644 --- a/src/qemu/qemu_process.c +++ b/src/qemu/qemu_process.c @@ -593,10 +593,14 @@ no_memory: static void qemuProcessHandleMonitorDestroy(qemuMonitorPtr mon, virDomainObjPtr vm) { - qemuDomainObjPrivatePtr priv = vm->privateData; + qemuDomainObjPrivatePtr priv; + + virDomainObjLock(vm); + priv = vm->privateData; if (priv->mon == mon) priv->mon = NULL; - virDomainObjUnref(vm); + if (virDomainObjUnref(vm) > 0) + virDomainObjUnlock(vm); } static qemuMonitorCallbacks monitorCallbacks = {