We can guess though. It isn't waiting for a lock - that would show in
the above list - so it might be waiting for a wakeup, or might be
spinning.
The only wake-up I can imagine is in one of the memory-allocation calls,
but if the system were running out of memory we would probably see
messages about that.
I wonder if it could be looping in svc_xprt_destroy_all(), and sitting
in the msleep() when the hang is detected so there are no locks to
report. I can't see while it would block there.
It would really help to get a full task list.
There is a sysctl for that: /proc/sys/kernel/hung_task_all_cpu_backtrace
Could that be enabled?