> kernel was working well, but when I tried to compile entire
> userland, the compilation hangs at the middle because a
> troff process became zombi and couldn't return.

I've seen this. To track it down I added options LOCKDEBUG to my kernel (and
a few more debug options) and installed a serial console - since then the
machine is running rock solid, complete a make build and complete rebuild of
gnome as well as heavy network traffic and any sorts of torture I could think
of.

I will start to remove debugging options and retry to crash it.

Martin