This question is about RedHat 7/8/9 alike.
After applying most updates, it is enough to restart the affected services.
RedHat provides the tracer tool to identify exactly what needs to be rebooted. It produces output similar to the following:
You should restart:
* Some applications using:
systemctl restart chronyd
systemctl restart firewalld
systemctl restart mariadb
systemctl restart polkit
systemctl restart postfix
systemctl restart rngd
systemctl restart rsyslog
systemctl restart sshd
systemctl restart sssd
systemctl restart systemd-journald
systemctl restart systemd-logind
systemctl restart systemd-udevd
systemctl restart tuned
systemctl restart vmtoolsd
* These applications rebooting your computer:
dbus
kernel
systemd
My understanding is that only kernel updates (without ksplice) should require a reboot.
How do I restart systemd and dbus without a reboot?
I already tried
systemctl daemon-reexec
To restart the PID-1 process.
But after tracer -a still reports that systemd "requires rebooting your computer".
For dbus, I don't even know where to start looking.