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The feature is constructed by two layers. One is generic interface, and
the other is exact implementation. This is rather historical. Originally
aufs implemented this feature based upon 'inotify.' Later 'fsnotify'
made 'inotify' obsolete. During the transition period, these two layers
were introduced to support both of 'inotify' and 'fsnotify.' Currently
only 'fsnotify' is supported, but the layers are kept for the future
use.

This feature is compiled only when CONFIG_AUFS_HNOTIFY is enabled.
See also the document in previous commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The branch object is managed by the sbinfo object as an element of its
internal array. The iinfo and dinfo objects contain the branch id, and
it will be used to implement the correct order in branch management
(add/del).

See also the documents in this commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The generation of aufs objects will be updated by the branch
management (add/del branches, by later commit), and aufs will refresh
the objects based upon the generation.
In refreshing dinfos, aufs will find all ancestors of the given dentry
and store the pointers in dynamically allocated memory. I don't think it
beautiful, but I don't have any other idea.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>