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The issue is that the threading implementation in Linux kernels 
previous to 2.6 diverged from the pthreads standard for signal 
handling. Normally signals are sent to the process and can be 
handled by any thread. In the LinuxThreads implementation of 
pthreads, signals are sent to a specific thread. If that thread 
blocks signals (which is what happens to all threads spawned in 
Python 2.2) then those signals do not get routed to a thread with 
them unblocked (what Python calls the "main thread")

The new threading facility in Linux 2.6, the NPTL, does not have 
this signal handling bug.

A simple python script that shows the problem is included below. 
This will hang in Linux kernels before 2.6 or RedHat customized 
kernels before RH9.

#!/usr/bin/python

import signal
import thread
import os

def handle_signals(sig, frame): pass
def send_signals(): os.kill(os.getpid(), signal.SIGSEGV)

signal.signal(signal.SIGSEGV, handle_signals)
thread.start_new_thread(send_signals, ())
signal.pause()