bug (?) with statvfs call on a subvolume

[redneb@gmx.com]

I am trying to use EncFS in conjunction with btrfs and I ran into a 
weird situation which I think is caused by statvfs being called on a 
btrfs subvolume.


In particular, when I do something like the following:
        statvfs("/mnt/btrfs-volume/some-subvolume")
then all mounted FUSE file-systems receive a GETATTR request. This is 
strange because it happens to FUSE file-systems completely unrelated to 
/mnt/btrfs-volume.


This can cause serious problems with some FUSE file-systems such as 
EncFS.  For instance, suppose that you have an EncFS file-systems as 
follows:
        encfs /mnt/btrfs-volume/some-subvolume /home/user/private
Whenever you do statvfs("/home/user/private") then EncFS will internally 
call statvfs("/mnt/btrfs-volume/some-subvolume") but this in turn will 
cause the FUSE method getattr to be called. This cycle of dependencies 
results in a deadlock.


STEP TO REPRODUCE:
1. btrfs subvolume create /mnt/btrfs-volume/private
2. mkdir /home/user/private
3. encfs /mnt/btrfs-volume/private /home/user/private
4. [ do statvfs("/home/user/private") ]

In step 4, stavfs will never return and /home/user/private will become 
unresponsive.