Re: [PATCH 0/2] printk vs rq->lock and xtime lock

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

On Fri, 2008-08-08 at 09:41 -0700, Linus Torvalds wrote:
> 
> On Fri, 8 Aug 2008, Peter Zijlstra wrote:
> > 
> > Something along the lines of the below patch?
> 
> Could we not literally just make this a RCU event? Unconditionally too?

Sure, but the RCU callback period is at least 3 jiffies and much longer
when busy - I'm not sure how long before we force a grace period, we do
that to avoid DoS, right Paul?

So this version would have a much higher risk of overflowing the console
buffer and making klogd miss bits. Then again, I really don't care about
klogd at _all_, I've been running with the wakeup patched out for ages.


Gah, the below doesn't boot - because I guess we start using rcu before
its properly set up.. should I poke at it more?

---
diff --git a/kernel/printk.c b/kernel/printk.c
index b51b156..3d80e30 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 +983,38 @@ int is_console_locked(void)
 	return console_locked;
 }
 
-void wake_up_klogd(void)
+void __wake_up_klogd(struct rcu_head *head);
+
+static struct {
+	struct rcu_head head;
+	spinlock_t	lock;
+	int		pending;
+} klogd_wakeup_state = {
+	.lock = __SPIN_LOCK_UNLOCKED(klogd_wakeup_state.lock),
+};
+
+void __wake_up_klogd(struct rcu_head *head)
 {
+	unsigned long flags;
+
+	spin_lock_irqsave(&klogd_wakeup_state.lock, flags);
+	BUG_ON(!klogd_wakeup_state.pending);
 	if (!oops_in_progress && waitqueue_active(&log_wait))
 		wake_up_interruptible(&log_wait);
+	klogd_wakeup_state.pending = 0;
+	spin_unlock_irqrestore(&klogd_wakeup_state.lock, flags);
+}
+
+void wake_up_klogd(void)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&klogd_wakeup_state.lock, flags);
+	if (!klogd_wakeup_state.pending) {
+		call_rcu(&klogd_wakeup_state.head, __wake_up_klogd);
+		klogd_wakeup_state.pending = 1;
+	}
+	spin_unlock_irqrestore(&klogd_wakeup_state.lock, flags);
 }
 
 /**