printk: robustify printk

Avoid deadlocks against rq->lock and xtime_lock by deferring the klogd
wakeup by polling from the timer tick.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/timer.c

diff --git a/kernel/timer.c b/kernel/timer.c
index 03bc7f1..510fe69 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -978,6 +978,7 @@
 	run_local_timers();
 	if (rcu_pending(cpu))
 		rcu_check_callbacks(cpu, user_tick);
+	printk_tick();
 	scheduler_tick();
 	run_posix_cpu_timers(p);
 }