Ah... okay. Stupid me. libata EH always resets a frozen port to
un-freeze it even if it's unoccupied to listen for hotplug events. So,
if dock notifies device removal after the actual device is gone && the
port is frozen as a result, libata EH will try to reset the port after
the device is gone and in this case the controller locks up the whole
machine for that. If schedule_eh is used, libata EH just removes the
device and does nothing else and the controller is happy.
This isn't too safe tho. There can be other things which can trigger
port reset. ie. hotplug request from userland, in-flight IOs at the
time of dock removal, etc... Maybe we need to implement a flag to
indicate that the port is dead and shouldn't be accessed in any way.