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/printk.c

diff --git a/kernel/printk.c b/kernel/printk.c
index b51b156..655cc2c 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -982,10 +982,25 @@
 	return console_locked;
 }
 
+static DEFINE_PER_CPU(int, printk_pending);
+
+void printk_tick(void)
+{
+	if (__get_cpu_var(printk_pending)) {
+		__get_cpu_var(printk_pending) = 0;
+		wake_up_interruptible(&log_wait);
+	}
+}
+
+int printk_needs_cpu(int cpu)
+{
+	return per_cpu(printk_pending, cpu);
+}
+
 void wake_up_klogd(void)
 {
-	if (!oops_in_progress && waitqueue_active(&log_wait))
-		wake_up_interruptible(&log_wait);
+	if (waitqueue_active(&log_wait))
+		__get_cpu_var(printk_pending) = 1;
 }
 
 /**

kernel/time/tick-sched.c

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 825b4c0..c13d4f1 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -255,7 +255,7 @@
 	next_jiffies = get_next_timer_interrupt(last_jiffies);
 	delta_jiffies = next_jiffies - last_jiffies;
 
-	if (rcu_needs_cpu(cpu))
+	if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu))
 		delta_jiffies = 1;
 	/*
 	 * Do not stop the tick, if we are only one off

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);
 }