mrr a écrit :
I saw somewhere during kernel compilation an option to load a kernel on
the fly but I haven't gone further (maybe it was targeted at kernel
developers).
If you're talking about kexec, it just avoid the firmware POST and
bootloader stage of a complete reboot, but not the system init. AFAIK
you cannot just swap kernels while userland processes keep running.
Or are you talking about live kernel patching such as ksplice, which
allows security patches to be applied to a running kernel without
rebooting ?
So usually, you do reboot after a kernel upgrade if you want to run the
new one.
It's the only occasion (I hope systemd hasn't changed that!) when an
upgrade needs a reboot (or am I wrong?). Sometimes though you need a
simple log out / log in.
All running processes using an upgraded executable must be restarted,
otherwise it keeps using the old version. This include all the libraries
it uses. When the libc, which is used by most programs, is upgraded, it
may be easier to reboot the whole system.