d_drop unhashes the entry from the parent dentry hashes, so that it won't be found through a VFS lookup any more. Note that this is different from deleting the dentry - d_delete will try to mark the dentry negative if possible, giving a successful _negative_ lookup, while d_drop will just make the cache lookup fail.
d_drop is used mainly for stuff that wants to invalidate a dentry for some reason (NFS timeouts or autofs deletes).
__d_drop requires dentry->d_lock.