Lustre 2.4.2 client on SL 6.x: recompile for older kernels.

It is fact that on cluster environments is not possible to keep linux kernel up to date.
For sure it is recommended time to time update kernels on all cluster if some critical security bugs are fixed.
Recently Lustre 2.4.2 has been released with kernel-2.6.32-358.23.2.el6.
If you run different kernel it is not a problem to recompile it.
In order to recompile
1) download src of client:
wget http://downloads.whamcloud.com/public/lustre/latest-maintenance-release/el6/client/SRPMS/lustre-client-2.4.2-2.6.32_358.23.2.el6.x86_64.src.rpm

2) rebuild using following command:

rpmbuild --rebuild --define 'lustre_name lustre-client' --define 'configure_args --disable-server' lustre-client-2.4.2-2.6.32_358.23.2.el6.x86_64.src.rpm

If you run this as root the new rpms will be generated in:
/root/rpmbuild/RPMS/x86_64

To install:
cd /root/rpmbuild/RPMS/x86_64 && yum install ./*.rpm

The last step reload lnet and lustre modules before remounting the lustre filesystem:
service lustre stop;service lnet stop;lustre_rmmod;service lnet stop;

Some times osc has still open references, therefore we need to   stop lnet twice.

Finally:
service lnet start;modprobe lustre;dmesg

the dmesg should contain some thing like:
Lustre: Lustre: Build Version: 2.4.2-RC2--PRISTINE-2.6.32-358.14.1.el6.x86_64

If you would like to test which versuon of client is running just check in:
cat /proc/fs/lustre/version

So we did it lustre client upgrade without rebooting. Have a fun with new lustre client.