When integrating pycairo into a django app pycairo, we discovered pycairo can deadlock the web server by holding the global interpreter lock.  This happens because, by default, mod_wsgi creates a 'subinterpreter' to run the application. For example, in a django view function do something like this:

from cStringIO import StringIO  # both StringIO and cStringIO cause the issue
import cairo

out = StringIO()
surface = cairo.SVGSurface(out, 10, 10)

surface.finish()  # deadlock

The offending lines of code appear to be in surface.c
static cairo_status_t
_write_func (void *closure, const unsigned char *data, unsigned int length) { ... }

I think the calls to PyGILState_Ensure and PyGILState_Release need to be replaced with calls to the Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS macros

WORKAROUND
set WSGIApplicationGroup %{GLOBAL} in the apache conf.
It's also a good idea to reduce the deadlock timeout of the WSGIDaemonProcess.