Currently, qemuProcessStop() unlocks given domain object right in
the middle of cleanup process. This is dangerous because there
might be another thread which is executing virDomainObjListAdd().
And since the domain object is on the list of domain objects AND
by the time qemuProcessStop() unlocks it the object is also
marked as inactive, the other thread acquires the lock and
switches vm->def pointer.

The unlocking of domain object is needed though, to allow even
processing thread finish its queue. Well, the processing can be
done before any cleanup is attempted.

Therefore, use freshly introduced virEventThreadStop() to join
the event thread and drop lock/unlock from the middle of
qemuProcessStop().

Fixes: 3865410e7f67ca4ec66e9a905e75f452762a97f0
Resolves: https://issues.redhat.com/browse/RHEL-49607
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_process.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index cec739c984..a07ce50fbf 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -8692,6 +8692,16 @@ void qemuProcessStop(virQEMUDriver *driver,
                                  VIR_QEMU_PROCESS_KILL_FORCE|
                                  VIR_QEMU_PROCESS_KILL_NOCHECK));
 
+    /* By unlocking the domain object the events processing thread is allowed
+     * to finish its job. Unlocking must happen before resetting vm->def->id as
+     * the global domain object list code depends on it (and it can't actually
+     * check 'priv->beingDestroyed as that's private). */
+    if (priv->eventThread) {
+        virObjectUnlock(vm);
+        virEventThreadStop(priv->eventThread);
+        virObjectLock(vm);
+    }
+
     if (priv->agent) {
         g_clear_pointer(&priv->agent, qemuAgentClose);
     }
@@ -8723,13 +8733,13 @@ void qemuProcessStop(virQEMUDriver *driver,
     vm->def->id = -1;
     priv->beingDestroyed = false;
 
+    /* No unlocking of @vm after this point until whole cleanup is done. */
+
     /* Wake up anything waiting on domain condition */
     virDomainObjBroadcast(vm);
 
-    /* IMPORTANT: qemuDomainObjStopWorker() unlocks @vm in order to prevent
-     * deadlocks with the per-VM event loop thread. This MUST be done after
-     * marking the VM as dead */
-    qemuDomainObjStopWorker(vm);
+    if (priv->eventThread)
+        g_object_unref(g_steal_pointer(&priv->eventThread));
 
     if (!!g_atomic_int_dec_and_test(&driver->nactive) && driver->inhibitCallback)
         driver->inhibitCallback(false, driver->inhibitOpaque);
-- 
2.44.2

On Wed, Aug 07, 2024 at 14:44:56 +0200, Michal Privoznik wrote:
> Currently, qemuProcessStop() unlocks given domain object right in
> the middle of cleanup process. This is dangerous because there
> might be another thread which is executing virDomainObjListAdd().
> And since the domain object is on the list of domain objects AND
> by the time qemuProcessStop() unlocks it the object is also
> marked as inactive, the other thread acquires the lock and
> switches vm->def pointer.
> 
> The unlocking of domain object is needed though, to allow even
> processing thread finish its queue. Well, the processing can be
> done before any cleanup is attempted.
> 
> Therefore, use freshly introduced virEventThreadStop() to join
> the event thread and drop lock/unlock from the middle of
> qemuProcessStop().
> 
> Fixes: 3865410e7f67ca4ec66e9a905e75f452762a97f0
> Resolves: https://issues.redhat.com/browse/RHEL-49607
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> ---

I've requested that the commit message carries an explanation why you
can change the code to contradict what the coment, which is being
deleted below states.

>  src/qemu/qemu_process.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index cec739c984..a07ce50fbf 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -8692,6 +8692,16 @@ void qemuProcessStop(virQEMUDriver *driver,
>                                   VIR_QEMU_PROCESS_KILL_FORCE|
>                                   VIR_QEMU_PROCESS_KILL_NOCHECK));
>  
> +    /* By unlocking the domain object the events processing thread is allowed
> +     * to finish its job. Unlocking must happen before resetting vm->def->id as
> +     * the global domain object list code depends on it (and it can't actually
> +     * check 'priv->beingDestroyed as that's private). */
> +    if (priv->eventThread) {

I've also requested to explicitly set 'priv->beingDestroyed' before
unlocking as qemuProcessStop may be called from code paths which don't
do that.

With both of the above addressed as I've requested in previous review:

Reviewed-by: Peter Krempa <pkrempa@redhat.com>