Updated rst docs to include threading model.

Change-Id: Id12cd6584dfd22fe32097cfddf0247c0cc115ac0

doc/source/devref/index.rst

@@ -35,6 +35,7 @@ Background Concepts for Nova
 .. toctree::
    :maxdepth: 3
 
+   threading
    distributed_scheduler
    multinic
    zone

doc/source/devref/threading.rst

@@ -0,0 +1,17 @@
+Threading model
+===============
+
+All OpenStack services use *green thread* model of threading, implemented 
+through using the Python `eventlet <http://eventlet.net/>`_ and 
+`greenlet <http://packages.python.org/greenlet/>`_ libraries.
+
+Green threads use a cooperative model of threading: thread context 
+switches can only occur when specific eventlet or greenlet library calls are 
+made (e.g., sleep, certain I/O calls). From the operating system's point of 
+view, each OpenStack service runs in a single thread. 
+
+The use of green threads reduces the likelihood of race conditions, but does
+not completely eliminate them. In some cases, you may need to use the 
+``@utils.synchronized(...)`` decorator to avoid races.
+
+