[PATCH] printk: robustify printk

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

Avoid deadlocks against rq->lock and xtime_lock by using RCU to defer
the klogd wakeup.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Steven Rostedt <srostedt@redhat.com>
---
diff --git a/kernel/printk.c b/kernel/printk.c
index b51b156..8870c2e 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -32,6 +32,7 @@
 #include <linux/security.h>
 #include <linux/bootmem.h>
 #include <linux/syscalls.h>
+#include <linux/rcupdate.h>
 
 #include <asm/uaccess.h>
 
@@ -982,10 +983,37 @@ int is_console_locked(void)
 	return console_locked;
 }
 
+struct klogd_wakeup_state {
+	struct rcu_head head;
+	int pending;
+};
+
+DEFINE_PER_CPU(struct klogd_wakeup_state, kws);
+
+void __wake_up_klogd(struct rcu_head *head)
+{
+	struct klogd_wakeup_state *kws = 
+		container_of(head, struct klogd_wakeup_state, head);
+
+	wake_up_interruptible(&log_wait);
+	kws->pending = 0;
+}
+
 void wake_up_klogd(void)
 {
-	if (!oops_in_progress && waitqueue_active(&log_wait))
-		wake_up_interruptible(&log_wait);
+	unsigned long flags;
+	struct klogd_wakeup_state *kws;
+
+	if (!waitqueue_active(&log_wait))
+		return;
+
+	local_irq_save(flags);
+	kws = &__get_cpu_var(kws);
+	if (!kws->pending) {
+		kws->pending = 1;
+		call_rcu(&kws->head, __wake_up_klogd);
+	}
+	local_irq_restore(flags);
 }
 
 /**