Not rebooting after a kernel upgrade usually means you keep running the old kernel until the next restart, so the new kernel is installed but not active yet. That can leave you missing bug fixes, security fixes, and kernel- side changes, and some new modules or hardware-related features may not work correctly until you reboot.

What actually happens

  • The new kernel files are on disk, but the running system is still using the previous kernel.
  • Kernel modules tied to the updated kernel may not load properly in the old running kernel.
  • Logging out and back in does not switch kernels; only a reboot does.

When it matters most

  • If the update includes security fixes, delaying the reboot delays those protections.
  • If you rely on drivers, encrypted volumes, USB devices, or DKMS modules, those may keep behaving like they are on the old kernel until reboot.
  • On some systems, services keep working fine for a while, which is why people sometimes postpone the reboot without immediate trouble.

Practical rule

Reboot after a kernel upgrade whenever you can, especially on desktops, laptops, and servers that need the newest fixes. If uptime is critical, you can delay it briefly, but you are still running the previous kernel until the restart.

Quick forum-style takeaway

“Installed” and “running” are different: the new kernel is installed first, then it becomes active only after reboot.

TL;DR: nothing instantly breaks just because you didn’t reboot, but the upgrade is incomplete until you do, and you may be missing fixes or new kernel behavior in the meantime.