[PATCH] qom: assert integer does not overflow

["Michael S. Tsirkin" <mst@redhat.com>]

QOM reference counting is not designed with an infinite amount of
references in mind, trying to take a reference in a loop will overflow
the integer. We will then eventually assert when dereferencing, but the
real problem is in object_ref so let's assert there to make such issues
cleaner and easier to debug.

Some micro-benchmarking shows using fetch and add this is essentially
free on x86.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 qom/object.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/qom/object.c b/qom/object.c
index 4f0677cca9..5db3974f04 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -1167,10 +1167,14 @@ GSList *object_class_get_list_sorted(const char *implements_type,
 Object *object_ref(void *objptr)
 {
     Object *obj = OBJECT(objptr);
+    uint32_t ref;
+
     if (!obj) {
         return NULL;
     }
-    qatomic_inc(&obj->ref);
+    ref = qatomic_fetch_inc(&obj->ref);
+    /* Assert waaay before the integer overflows */
+    g_assert(ref < INT_MAX);
     return obj;
 }
 
-- 
MST