Problem Description:
The kernel holds a lock over the source directory vnode
while trying to convert the target directory file handle
to a vnode, which needs to be returned with the lock held,
too. This order may be in violation of normal lock order,
which in conjunction with other threads that grab locks in
the right order, constitutes a deadlock condition because
no thread can proceed.
Impact:
An attacker on a trusted client could cause the NFS
server become deadlocked, resulting in a denial of service.