I believe that CoreOS can use kexec to update the kernel. kexec basically looks the same as a reboot except instead of stopping fully, and going through the BIO

I believe that CoreOS can use kexec to update the kernel. kexec basically looks the same as a reboot except instead of stopping fully, and going through the BIOS/bootloader again (which can take a long time on server-grade hardware), the old kernel 'execs' the new kernel and boots userspace back up. https://en.wikipedia.org/wiki/Kexec https://en.wikipedia.org/wiki/Kexec