Correction - 4.4.0-116-generic does not solve the problem.
The server was up for over an hour before hanging. As it doesn't usually have a console, I can't be certain of the cause of the hang. However, a subsequent reboot, with a console, involving rebuilding a 10TB RAID 5 array, showed exactly the same Watchdog Hard Lockup for each core during the boot process.
Correction - 4.4.0-116-generic does not solve the problem.
The server was up for over an hour before hanging. As it doesn't usually have a console, I can't be certain of the cause of the hang. However, a subsequent reboot, with a console, involving rebuilding a 10TB RAID 5 array, showed exactly the same Watchdog Hard Lockup for each core during the boot process.
Now running with noibpb again.