sched/null: avoid crash after failed domU creation

When creating a domU, but the creation fails, there is a corner case that may
lead to a crash in the null scheduler when running a debug build of Xen.

(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Assertion 'npc->unit == unit' failed at common/sched/null.c:379
(XEN) ****************************************

The events leading to the crash are:

* null_unit_insert() was invoked with the unit offline. Since the unit was
  offline, unit_assign() was not called, and null_unit_insert() returned.
* Later during domain creation, the unit was onlined
* Eventually, domain creation failed due to bad configuration
* null_unit_remove() was invoked with the unit still online. Since the unit was
  online, it called unit_deassign() and triggered an ASSERT.

To fix this, only call unit_deassign() when npc->unit is non-NULL in
null_unit_remove.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Dario Faggioli <dfaggioli@suse.com>

xen/common/sched/null.c

@@ -522,6 +522,8 @@ static void cf_check null_unit_remove(
 {
     struct null_private *prv = null_priv(ops);
     struct null_unit *nvc = null_unit(unit);
+    struct null_pcpu *npc;
+    unsigned int cpu;
     spinlock_t *lock;
 
     ASSERT(!is_idle_unit(unit));
@@ -531,8 +533,6 @@ static void cf_check null_unit_remove(
     /* If offline, the unit shouldn't be assigned, nor in the waitqueue */
     if ( unlikely(!is_unit_online(unit)) )
     {
-        struct null_pcpu *npc;
-
         npc = unit->res->sched_priv;
         ASSERT(npc->unit != unit);
         ASSERT(list_empty(&nvc->waitq_elem));
@@ -549,7 +549,10 @@ static void cf_check null_unit_remove(
         goto out;
     }
 
-    unit_deassign(prv, unit);
+    cpu = sched_unit_master(unit);
+    npc = get_sched_res(cpu)->sched_priv;
+    if ( npc->unit )
+        unit_deassign(prv, unit);
 
  out:
     unit_schedule_unlock_irq(lock, unit);