When updating the package gdm on my machine running with FACTORY, rpm hangs up at %post.  Looking at the process list, it stalls while running /usr/sbin/usermod like below:
   /usr/sbin/usermod -g gdm -G gdm -s /bin/false gdm
I have to send KILL signal (TERM doesn't help) to usermod to proceed the installation.

Indeed, when I invoke the above usermod command manually, it hangs up.  The gdb shows the following point:

#0  0x00007ffff6cac7e0 in read () from /lib64/libc.so.6
#1  0x00007ffff6f439bf in nscd_flush_cache () from /lib64/libnscd.so.1
#2  0x00000000004083f9 in main (argc=<value optimized out>, 
    argv=<value optimized out>) at usermod.c:1517
(gdb) up
#1  0x00007ffff6f439bf in nscd_flush_cache () from /lib64/libnscd.so.1
(gdb) up
#2  0x00000000004083f9 in main (argc=<value optimized out>, 
    argv=<value optimized out>) at usermod.c:1517
1517	      nscd_flush_cache ("passwd");

So, this can be an issue of libnscd, but I have no idea right now.