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All the file operations have the following locking order - parent,
child. If a lock for a child is returned to the client, the following
operations on this file are done by the child fid.
However, the migrate is an exception - it takes the lease lock first and
takes the PW parent lock next during the MDS_REINT.
At the same time, if there is a parallel racing operation (open) which
has taken a lock on parent (conflicting with the next MDS_REINT) and
is trying to take a lock on child - it is blocked until
the lease cancel comes.
The lease cancel is piggy-backed on the MDS_REINT RPC and is handled
at the end of the operation, trying to take the conflicting parent lock
first - thus a deadlock occurs.
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