op is a KeyedRef instance.  The instance being cleared from the module
is the multiprocessing.util._afterfork_registry.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7d626b0 (LWP 2287)]
0x0809a131 in _Py_ForgetReference (op=0xb7a9a814) at Objects/object.c:2022
2022            if (op == &refchain ||
(gdb) bt
#0  0x0809a131 in _Py_ForgetReference (op=0xb7a9a814) at
Objects/object.c:2022
#1  0x0809a1a0 in _Py_Dealloc (op=0xb7a9a814) at Objects/object.c:2043
#2  0x080b436e in tupledealloc (op=0xb79ad1f4) at Objects/tupleobject.c:169
#3  0x0809a1ab in _Py_Dealloc (op=0xb79ad1f4) at Objects/object.c:2044
#4  0x08065bdf in PyObject_CallFunctionObjArgs (callable=0xb79baa84)
    at Objects/abstract.c:2716
#5  0x080cabc4 in handle_callback (ref=0xb7a9a814, callback=0xb79baa84)
    at Objects/weakrefobject.c:864
#6  0x080cad6e in PyObject_ClearWeakRefs (object=0xb79bd624)
    at Objects/weakrefobject.c:910
#7  0x08168971 in func_dealloc (op=0xb79bd624) at Objects/funcobject.c:453
#8  0x0809a1ab in _Py_Dealloc (op=0xb79bd624) at Objects/object.c:2044
#9  0x080b436e in tupledealloc (op=0xb79a65f4) at Objects/tupleobject.c:169
#10 0x0809a1ab in _Py_Dealloc (op=0xb79a65f4) at Objects/object.c:2044
#11 0x080b7c26 in clear_slots (type=0x82af4e4, self=0xb7a9a814)
    at Objects/typeobject.c:821
#12 0x080b806e in subtype_dealloc (self=0xb7a9a814) at
Objects/typeobject.c:950
#13 0x0809a1ab in _Py_Dealloc (op=0xb7a9a814) at Objects/object.c:2044
#14 0x080915b2 in dict_dealloc (mp=0xb79b9674) at Objects/dictobject.c:907
#15 0x0809a1ab in _Py_Dealloc (op=0xb79b9674) at Objects/object.c:2044
#16 0x080915b2 in dict_dealloc (mp=0xb79b9494) at Objects/dictobject.c:907
#17 0x0809a1ab in _Py_Dealloc (op=0xb79b9494) at Objects/object.c:2044
#18 0x08068720 in instance_dealloc (inst=0xb79b6edc)
    at Objects/classobject.c:668
#19 0x0809a1ab in _Py_Dealloc (op=0xb79b6edc) at Objects/object.c:2044
#20 0x08090517 in insertdict (mp=0xb79a5b74, key=0xb7a9ae38,
hash=-1896994012, 
    value=0x81bdd6c) at Objects/dictobject.c:455
#21 0x08090da6 in PyDict_SetItem (op=0xb79a5b74, key=0xb7a9ae38, 
    value=0x81bdd6c) at Objects/dictobject.c:697
#22 0x08095ad3 in _PyModule_Clear (m=0xb7a88334) at
Objects/moduleobject.c:125
#23 0x08111443 in PyImport_Cleanup () at Python/import.c:479
#24 0x08120cb3 in Py_Finalize () at Python/pythonrun.c:430
#25 0x0805b618 in Py_Main (argc=1, argv=0xbfbaf434) at Modules/main.c:623
#26 0x0805a2e6 in main (argc=0, argv=0x0) at ./Modules/python.c:23