hung task debugging: Inject NMI when hung and going to panic

Send an NMI to all CPUs when a hung task is detected and the hung
task code is configured to panic. This gives us a fairly uptodate
snapshot of all CPUs in the system.

This lets us get stack trace of all CPUs which makes life easier
trying to debug a deadlock, and the NMI doesn't change anything
since the next step is a kernel panic.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1331848040-1676-1-git-send-email-levinsasha928@gmail.com
[ extended the changelog a bit ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/hung_task.c

@@ -108,8 +108,10 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 
 	touch_nmi_watchdog();
 
-	if (sysctl_hung_task_panic)
+	if (sysctl_hung_task_panic) {
+		trigger_all_cpu_backtrace();
 		panic("hung_task: blocked tasks");
+	}
 }
 
 /*