Missing documentation

In my time with Linux there are some things that would benefit from better documentation.

1. The use of per-inode mutexes for buffered IO writes. This prevents concurrent writes per file between the block layer and storage. The problem is most easy to see with a hard disk when the drive write cache is disabled. XFS with O_DIRECT avoids this problem. I am not sure about other file systems with O_DIRECT. TIL the per-inode mutex is now the per-inode rwsem. An old FB note on this is here. A paper on filesystem scalability is here.
2. PTHREAD_MUTEX_ADAPTIVE_NP - this enables some busy-waiting when trying to lock a mutex. It isn't mentioned in man pages. Interesting details are here.