Big Kernel Lock
The Big Kernel Lock is the most annoying lock in the kernel. It was introduced to start with the SMP development of Linux more than 10 years ago. We still have users which rely on it. There has been some effort to push the BKL locking down into the files which (still) expect to be protected by it for whatever reason. This makes it easier to replace it by local locking.
Some effort to remove the BKL locking in various places has already happened. The most prominent work is the removal of the BKL from reiserfs by Frederic Weisbecker. This work and other bits an pieces can be found in the kill-the-BKL branch of the -tip git repository. Please check that first if you want to help with that effort. Secondly please add the file(s) you are working on to that page to avoid duplicate effort.