This could be K4 related, to reproduce:

1) Install latest master K4 controller from nexus:
https://nexus.opendaylight.org/content/repositories/opendaylight.snapshot/org/opendaylight/integration/karaf/0.7.0-SNAPSHOT/

2) Install feature: odl-openflowplugin-flow-services-rest

3) Start mininet 1 switch: sudo mn --controller=remote,ip=192.168.0.1 --topo linear,1

4) Do GET http://192.168.0.1:8181/restconf/operational/network-topology:network-topology/ using HTTP headers: Accept=application/xml, Content-Type=application/xml

Controller hangs, only soultion is restart controller.