Comment The explanation, deadlock...do kill the messenger (Score 1) 284
Simple version:
"dont kill the messenger" except when the messenger is going to kill you. Its printk sending notice that the leap second happened that deadlocks against the timer doing the leap second (both vying for xtime_lock). Call it a "feature" of the NTP code. Hence the "turn off NTPD" workaround, if NTP doesnt get notified it should implement the leap second from somewhere upstream, it wont notify about it to the kernel, and the printk shouldnt happen.
-T