Q: I get this message compiling RTAI:
/usr/include/asm/atomic.h:40:2: warning: #warning Using kernel header in userland program. BAD!
A: This is pre rtai-24.1.11. This means what it says: RTAI is picking up kernel headers when compiling. The old fix was to change the makefile in the offending places --I think it was in the usposix directory. In any case, this doesn't happen anymore with rtai3.x . It is recommended to upgrade anyways.
A1: Try loading the modules manually, insmod rtai.o etc.
A2: Replace rt_modprobe with Adam Taylor's script (https://mail.rtai.org/pipermail/rtai/2003-June/004022.html) to successfully solve the rt_modprobe problem. This is a perl script that is slightly different than the one that comes with the distribution. This was successful on a clean 2.4.20 rthal patched kernel (e.g., kernel from www.kernel.org, not the Redhat kernel) and RTAI 24.1.11 on a system otherwise loaded with Redhat 9.