The backend may call update_latency() while at the same time
the auto-connect-port calls set_worst_io_latencies().
The latter already holds the process-lock, so update_latency()
first needs to acquire it, as well. If one already holds the
_update_latency_lock, one must not ask for the process-lock.
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Previously Ardour's connection manager first took the process_lock
and then waited to the _update_latency_lock.
Meanwhile jack calls latency_callback(), takes the
_update_latency_lock and waits for the process_lock.
Classic deadlock.
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