The calling thread will block until a read-lock is available (no thread is holding the global write-lock). Multiple threads may hold global read-locks at the same time. While holding a read-lock it is safe to traverse the scene graph and to query values of fields, but not to modify the scene graph (see method OIV.Inventor.SoDB.Writelock()). See OIV.Inventor.SoDB.Readunlock().

Reference