Notice about copy_from/to_user

     Because product schedule , I try to tidy out some useful message about kernel coding . Maybe we should change the ugly code.

this article is about proc system's copy_from/to_user:

1 . they should not be called in atomic context.
2 . you can use the atomic version (__copy_[to|from]_user) instead when in atomic. at the same time , you must check  user address is valid with access_ok in user context.
3 . before processing the user data in write_proc, we'd better copy them into kernel context with copy_from_user.