Index: Objects/weakrefobject.c
===================================================================
--- Objects/weakrefobject.c	(revision 64224)
+++ Objects/weakrefobject.c	(working copy)
@@ -907,7 +907,8 @@
             current->wr_callback = NULL;
             clear_weakref(current);
             if (callback != NULL) {
-                handle_callback(current, callback);
+                if(current->ob_refcnt > 0)
+                    handle_callback(current, callback);
                 Py_DECREF(callback);
             }
         }
@@ -925,9 +926,15 @@
             for (i = 0; i < count; ++i) {
                 PyWeakReference *next = current->wr_next;
 
-                Py_INCREF(current);
-                PyTuple_SET_ITEM(tuple, i * 2, (PyObject *) current);
-                PyTuple_SET_ITEM(tuple, i * 2 + 1, current->wr_callback);
+                if(current->ob_refcnt > 0)
+                {
+                    Py_INCREF(current);
+                    PyTuple_SET_ITEM(tuple, i * 2, (PyObject *) current);
+                    PyTuple_SET_ITEM(tuple, i * 2 + 1, current->wr_callback);
+                }
+                else {
+                    Py_DECREF(current->wr_callback);
+                }
                 current->wr_callback = NULL;
                 clear_weakref(current);
                 current = next;
@@ -935,6 +942,7 @@
             for (i = 0; i < count; ++i) {
                 PyObject *callback = PyTuple_GET_ITEM(tuple, i * 2 + 1);
 
+                /* The tuple may have slots left to NULL */
                 if (callback != NULL) {
                     PyObject *item = PyTuple_GET_ITEM(tuple, i * 2);
                     handle_callback((PyWeakReference *)item, callback);
Index: Lib/test/test_weakref.py
===================================================================
--- Lib/test/test_weakref.py	(revision 64224)
+++ Lib/test/test_weakref.py	(working copy)
@@ -666,7 +666,7 @@
         w = Target()
 
 
-class SubclassableWeakrefTestCase(unittest.TestCase):
+class SubclassableWeakrefTestCase(TestBase):
 
     def test_subclass_refs(self):
         class MyRef(weakref.ref):
@@ -730,7 +730,43 @@
         self.assertEqual(r.meth(), "abcdef")
         self.failIf(hasattr(r, "__dict__"))
 
+    def _test_subclass_refs_with_cycle(self):
+        # Bug #3110
+        # An instance of a weakref subclass can have attributes.
+        # If such a weakref holds the only strong reference to the object,
+        # deleting the weakref will delete the object. In this case,
+        # the callback must not be called, because the ref object is being deleted.
+        class MyRef(weakref.ref):
+            pass
 
+        # Use a local callback, for "regrtest -R::" to detect refcounting problems
+        def callback(w):
+            pass
+
+        o = C()
+        r1 = MyRef(o, callback)
+        r1.o = o
+        del o
+
+        del r1 # Used to crash here
+
+        self.assertEqual(self.cbcalled, 0)
+
+        # Same test, with two weakrefs to the same object
+        # (since code paths are different)
+        o = C()
+        r1 = MyRef(o, callback)
+        r2 = MyRef(o, callback)
+        r1.r = r2
+        r2.o = o
+        del o
+        del r2
+
+        del r1 # Used to crash here
+
+        self.assertEqual(self.cbcalled, 0)
+
+
 class Object:
     def __init__(self, arg):
         self.arg = arg
@@ -1171,6 +1207,7 @@
         MappingTestCase,
         WeakValueDictionaryTestCase,
         WeakKeyDictionaryTestCase,
+        SubclassableWeakrefTestCase,
         )
     test_support.run_doctest(sys.modules[__name__])