[PATCH] vm : forbid to start a being removed vm

[Hogan Wang]

> On 3/11/21 2:13 AM, Hogan Wang wrote:
> > From: Zhuang Shengen <zhuangshengen at huawei.com>
> > 
> > When a vm is doing migration phase confirm, and then start it 
> > concurrently, it will lead to the vm out of libvirtd control.
> > 
> > Cause Analysis:
> > 1. thread1 migrate vm out.
> > 2. thread2 start the migrating vm.
> > 3. thread1 remove vm from domain list after migrate success.
> 
> I guess I'm puzzled here. Thread1 has started migration or finished too? 
> How can thread2 start the VM during migration? Is that an offline migration?
> 
> To me it seems like there's a problem with what jobs are allowed during migration, or their order. Looking at qemuMigrationSrcConfirm() which is the function that does final step of migration (at the source), so it ends migration job, then it starts MODIFY job (which potentially unlocks the domain object) and only then it sets vm->removing = true and removes it from the list.
> 
> What I am trying to say is that because the vm object can be briefly unlocked thus another thread might lock it successfully, proceed to BeginJob(), where it unlocks the object again only to let the first thread to remove it from the list.
> 
> It's not only virDomainCreateWithFlags() that's affected - basically any other API that grabs a job. Even though, plenty of them require domain to be running, therefore they exit early with "domain not running" error message.
> 
> I wonder whether something among these lines fixes the problem for you (compile tested only, but it shows the idea):
>

Thanks for your suggestions, your proposal can not fix this issue, the reason is that virDomainCreateWithFlags task will finish the job after qemuMigrationSrcConfirm task remove the vm object from the domain list, and then qemu process still started however libvirtd lost the vm object.

I think qemuDomainObjBeginJobInternal should return error when some task acquire job for a removed vm object(vm->removing is true).

diff --git a/src/qemu/qemu_domainjob.c b/src/qemu/qemu_domainjob.c
index 50cfc45f5b..246c3208fc 100644
--- a/src/qemu/qemu_domainjob.c
+++ b/src/qemu/qemu_domainjob.c
@@ -880,6 +880,9 @@ qemuDomainObjBeginJobInternal(virQEMUDriverPtr driver,
    if (!nested && !qemuDomainNestedJobAllowed(&priv->job, job))
        goto retry;

+    if (obj->removing)
+        goto error;
+
    ignore_value(virTimeMillisNow(&now));

    if (job) {
@@ -1011,6 +1014,10 @@ qemuDomainObjBeginJobInternal(virQEMUDriverPtr driver,
                             "due to max_queued limit"));
        }
        ret = -2;
+    } else if (obj->removing) {
+        virReportError(VIR_ERR_OPERATION_FAILED, "%s",
+                       _("cannot acquire job mutex for removed domain"));
+        ret = -2;
    } else {
        virReportSystemError(errno, "%s", _("cannot acquire job mutex"));
    }
--

> diff --git i/src/qemu/qemu_migration.c w/src/qemu/qemu_migration.c index 79dcb4a15d..4b1bb77155 100644
> --- i/src/qemu/qemu_migration.c
> +++ w/src/qemu/qemu_migration.c
> @@ -3468,6 +3468,7 @@ qemuMigrationSrcConfirm(virQEMUDriverPtr driver,
>       qemuMigrationJobPhase phase;
>       g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
>       int ret = -1;
> +    bool haveJob;
> 
>       if (!qemuMigrationJobIsActive(vm, QEMU_ASYNC_JOB_MIGRATION_OUT))
>           goto cleanup;
> @@ -3485,15 +3486,21 @@ qemuMigrationSrcConfirm(virQEMUDriverPtr driver,
>                                          cookiein, cookieinlen,
>                                          flags, cancelled);
> 
> -    qemuMigrationJobFinish(driver, vm);
> +    haveJob = qemuDomainObjBeginJob(driver, vm, QEMU_JOB_MODIFY) >= 0;
> +
>       if (!virDomainObjIsActive(vm)) {
>           if (!cancelled && ret == 0 && flags & VIR_MIGRATE_UNDEFINE_SOURCE) {
>               virDomainDeleteConfig(cfg->configDir, cfg->autostartDir, vm);
>               vm->persistent = 0;
>           }
> -        qemuDomainRemoveInactiveJob(driver, vm);
> +        qemuDomainRemoveInactive(driver, vm);
>       }
> 
> +    if (haveJob)
> +        qemuDomainObjEndJob(driver, vm);
> +
> +    qemuMigrationJobFinish(driver, vm);
> +
>    cleanup:
>       virDomainObjEndAPI(&vm);
>       return ret;
> 
> 
> 
> > 4. thread2 acquired the vm job success and start the vm.
> > 5. cannot find the vm any more by 'virsh list' command. Actually,
> >     the started vm is not exist in the domain list.
> > 
> > Solution:
> > Check the vm->removing state before start.
> > 
> > 
> > Signed-off-by: Zhuang Shengen <zhuangshengen at huawei.com>
> > Reviewed-by: Hogan Wang <hogan.wang at huawei.com>
> > ---
> >   src/qemu/qemu_driver.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> > 
> > diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c index 
> > d1a3659774..a5dfea94cb 100644
> > --- a/src/qemu/qemu_driver.c
> > +++ b/src/qemu/qemu_driver.c
> > @@ -6637,6 +6637,12 @@ qemuDomainCreateWithFlags(virDomainPtr dom, unsigned int flags)
> >           goto endjob;
> >       }
> >   
> > +    if (vm->removing) {
> > +        virReportError(VIR_ERR_OPERATION_INVALID,
> > +                       "%s", _("domain is already removing"));
> 
> s/removing/being removed/
> 
> > +        goto endjob;
> > +    }
> > +
> >       if (qemuDomainObjStart(dom->conn, driver, vm, flags,
> >                              QEMU_ASYNC_JOB_START) < 0)
> >           goto endjob;
> > 
> 
> Michal