Re: [libvirt] [PATCH v2] qemu_migration: Avoid crashing if domain dies too quickly

18 Oct 2013


      I'm sorry. I didn't get what you mean.

In virQEMUCapsInitQMP

    if (!(xmlopt = virDomainXMLOptionNew(NULL, NULL, NULL)) ||
        !(vm = virDomainObjNew(xmlopt)))
        goto cleanup;

    vm->pid = pid;   //Apparently vm is not NULL here.

    if (!(mon = qemuMonitorOpen(vm, &config, true, &callbacks, NULL))) {  //If qemuMonitorOpen returns NULL here, but not do mon->vm = virObjectRef(vm); in qemuMonitorOpenInternal.
        ret = 0;
        goto cleanup;  // We go to cleanup here.
    }

    virObjectLock(mon);

    if (virQEMUCapsInitQMPMonitor(qemuCaps, mon) < 0)
        goto cleanup;

    ret = 0;

cleanup:
    if (mon)
        virObjectUnlock(mon);
    qemuMonitorClose(mon);
    virCommandAbort(cmd);
    virCommandFree(cmd);
    VIR_FREE(monarg);
    VIR_FREE(monpath);
    virObjectUnref(vm);    //vm is not NULL here, and we'll do something about vm->refs, right?
    virObjectUnref(xmlopt);
...
-----Original Message-----
From: Michal Privoznik [mailto:mprivozn@redhat.com]
Sent: Friday, October 18, 2013 1:12 PM
To: Wangyufei (A)
Cc: libvir-list@redhat.com; Wangrui (K)
Subject: Re: [libvirt] [PATCH v2] qemu_migration: Avoid crashing if domain
dies too quickly
On 18.10.2013 06:06, Wangyufei (A) wrote:
...
Thanks at first, this patch some kinda solve my problem until now. But I still
have a doubt about this patch.
...
-----Original Message-----
From: libvir-list-bounces@redhat.com
[mailto:libvir-list-bounces@redhat.com] On Behalf Of Michal Privoznik
Sent: Friday, October 11, 2013 8:15 PM
To: libvir-list@redhat.com
Subject: [libvirt] [PATCH v2] qemu_migration: Avoid crashing if domain
dies
too quickly
...
...
@@ -2673,6 +2677,8 @@ cleanup:
     virCommandFree(cmd);
     VIR_FREE(monarg);
     VIR_FREE(monpath);
+    virObjectUnref(vm);
Is this virObjectUnref(vm) corresponding to mon->vm = virObjectRef(vm)
added in qemuMonitorOpenInternal?
If it is, how can we confirm virObjectRef(vm) has been done in
qemuMonitorOpenInternal? If an error (anyone follows)happened in
qemuMonitorOpenInternal is before mon->vm = virObjectRef(vm),
then we still goto cleanup and do virObjectUnref(vm), the refs will be
wrong. Am I right?
Unfortunately, you've cut off the chunk above that allocates @mon.
Anyway, on initialization, @mon is filled with zeros. So until somebody
sets mon->vm [1] mon->vm is effectively NULL. And virObjectUnref() acts
like NOP on NULL.
...
if (!cb->eofNotify) {
        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                       _("EOF notify callback must be supplied"));
        return NULL;
    }
    if (!cb->errorNotify) {
        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                       _("Error notify callback must be supplied"));
        return NULL;
    }
if (qemuMonitorInitialize() < 0)
        return NULL;
if (!(mon = virObjectLockableNew(qemuMonitorClass)))
        return NULL;
mon->fd = -1;
    mon->logfd = -1;
    if (virCondInit(&mon->notify) < 0) {
        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                       _("cannot initialize monitor condition"));
        goto cleanup;
    }
    mon->fd = fd;
    mon->hasSendFD = hasSendFD;
    mon->vm = virObjectRef(vm);
1: ^^ until after this line
...
...
+    virObjectUnref(xmlopt);
if (pid != 0) {
         char ebuf[1024];
I hope it makes things a bit clearer.
Michal

    