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>

3 files changed, 19 insertions, 3 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index b51b1567bb55..655cc2ca10cc 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -982,10 +982,25 @@ int is_console_locked(void)
         return console_locked;
 }
 
-void wake_up_klogd(void)
+static DEFINE_PER_CPU(int, printk_pending);
+
+void printk_tick(void)
 {
-        if (!oops_in_progress && waitqueue_active(&log_wait))
+        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 (waitqueue_active(&log_wait))
+                __get_cpu_var(printk_pending) = 1;
 }
 
 /**
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 825b4c00fe44..c13d4f182370 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -255,7 +255,7 @@ void tick_nohz_stop_sched_tick(int inidle)
         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
diff --git a/kernel/timer.c b/kernel/timer.c
index 03bc7f1f1593..510fe69351ca 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -978,6 +978,7 @@ void update_process_times(int user_tick)
         run_local_timers();
         if (rcu_pending(cpu))
                 rcu_check_callbacks(cpu, user_tick);
+        printk_tick();
         scheduler_tick();
         run_posix_cpu_timers(p);
 }