what does the function wrapfs_d_revalidate and wrapfs_d_release do ??
(referring the copyright code at wrapfs.filesystems.org)
I do not know.
Just wanted to comment a bit on the question. wrapfs is meant to help developers writing other filesystems. It is opensource.
Developing a filesystem or device driver is not a trivial exercise. So. By asking this question the way you did, you raise another question - are you attempting to do something you may not be ready to tackle? Not that you should not try, because you will learn a lot. It is more because people will pick something to work on that is foreign enough to their skill set that it kills their desire to continue programming or working with computers.
I would suggest that, if you have not done so already, consider some reading.
This book is central to programming UNIX/Linux:
Rago and Stevens 'Advanced Programming in the UNIX Environment'
The Linux-only version of Rago: Michael Kerrisk 'The Linux Programming Interface'
Next step: Steve Pate 'UNIX Filesystems' This has a large section on Linux filesystems and user/kernel structures
Do not give up and have fun.