There is no need to reboot or worry.

Ureadahead, actually called über-readahead, is a system to speed up boot times by preloading boot files into pages and memory so they are ready immediately while called upon, via pack files which are already pre-packed into memory-friendly pages. It will generate a plan for preload monthly, and when files related to boot are modified, added, or removed. Since it only affects boot performance, it is not necessary to reboot since ureadahead only applies during boot, and the boot packing will be done on next boot anyway. However, it will sometimes run even when boot files or the monthly cycle don't apply.

How it actually works:

ureadahead is an executable that will trace the boot process, and then generate pack files. When run without arguments, this executable checks /var/lib/ureadahead for packfiles and if they are old enough, or don't exist, it begins tracing forever. It can then be given a SIGTERM with Ctrl+C, at which time it will save any packfiles. It can also be run with --timeout to make it automatically stop and save its packfiles.

Ref: https://wiki.archlinux.org/index.php/Ureadahead