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There's a three-process deadlock involving shared/exclusive barriers
and inverted lock orders in the directory readdir implementation.
It's a pre-existing problem with lock ordering, exposed by the
VFS parallelisation code.

process 1               process 2               process 3
---------               ---------               ---------
readdir
iolock(shared)
  get_leaf_dents
    iterate entries
       ilock(shared)
       map, lock and read buffer
       iunlock(shared)
       process entries in buffer
       .....
                                                readdir
                                                iolock(shared)
                                                  get_leaf_dents
                                                    iterate entries
                                                      ilock(shared)
                                                      map, lock buffer
                                                      ...