xen/sched: Fix the sorted insertion in sched_domain_insert_unit()

The loop links the new unit in front of the unit whose successor has a
higher unit_id, rather than after it: inserting 1 into {0, 2} gives
{1, 0, 2}. Stop at the first unit with a higher unit_id instead.

This goes unnoticed as long as units are inserted in unit_id order, as
guest vCPUs typically are.

Also rename prev_unit to next_unit: the new unit is linked in front of
*next_unit, not after it.

Fixes: 141db7fd4d ("xen/sched: build a linked list of struct sched_unit")
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

xen/common/sched/core.c

@@ -483,18 +483,17 @@ static struct sched_unit *sched_alloc_unit_mem(void)
 
 static void sched_domain_insert_unit(struct sched_unit *unit, struct domain *d)
 {
-    struct sched_unit **prev_unit;
+    struct sched_unit **next_unit;
 
     unit->domain = d;
 
-    for ( prev_unit = &d->sched_unit_list; *prev_unit;
+    for ( next_unit = &d->sched_unit_list; *next_unit;
-          prev_unit = &(*prev_unit)->next_in_list )
+          next_unit = &(*next_unit)->next_in_list )
-        if ( (*prev_unit)->next_in_list &&
+        if ( (*next_unit)->unit_id > unit->unit_id )
-             (*prev_unit)->next_in_list->unit_id > unit->unit_id )
             break;
 
-    unit->next_in_list = *prev_unit;
+    unit->next_in_list = *next_unit;
-    *prev_unit = unit;
+    *next_unit = unit;
 }
 
 static struct sched_unit *sched_alloc_unit(struct vcpu *v)