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SubjectRe: [PATCH 0/2] printk vs rq->lock and xtime lock
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On Fri, 2008-08-08 at 10:25 -0700, Linus Torvalds wrote:
>
> On Fri, 8 Aug 2008, Peter Zijlstra wrote:
> >
> > 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?
>
> I really don't think it matters. klogd is going to write the thing to
> _disk_ (or network), and three jiffies really don't matter. If we can fill
> the buffer in that kind of time, we're screwed for other reasons anyway.
>
> > 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.
>
> Well, I'd care a _bit_ about klogd, but not enough to worry about a couple
> of jiffies. We want to wake it up at some point, but...
>
> > Gah, the below doesn't boot - because I guess we start using rcu before
> > its properly set up.. should I poke at it more?
>
> I'd certainly prefer this kind of approach. However, may I suggest:
>
> - doing the "waitqueue_active(&log_wait)" before even bothering to do the
> RCU call. That, btw, will automatically mean that we wouldn't ever call
> the RCU code before anything is initialized.
>
> - get rid of the "oops_in_progress" thing, since I think the whole point
> of that was to avoid getting the lock recursively in the first place.
>
> - I'd worry about the "spin_lock_irqsave(&klogd_wakeup_state.lock)". What
> if the printk happens from call_rcu()? This is exactly what we're
> trying to get away from - having some parts of the kernel not able to
> printk() because of subtle locking issues.
>
> For that last thing, maybe we can just make it a percpu thing and just
> disable irq's?

You're _so_ right! :-)

This is much prettier and boots to boot.


Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
diff --git a/kernel/printk.c b/kernel/printk.c
index b51b156..10830d8 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 +983,43 @@ int is_console_locked(void)
return console_locked;
}

+void __wake_up_klogd(struct rcu_head *head);
+
+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)
+{
+ unsigned long flags;
+ struct klogd_wakeup_state *kws =
+ container_of(head, struct klogd_wakeup_state, head);
+
+ local_irq_save(flags);
+ BUG_ON(!kws->pending);
+ wake_up_interruptible(&log_wait);
+ kws->pending = 0;
+ local_irq_restore(flags);
+}
+
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) {
+ call_rcu(&kws->head, __wake_up_klogd);
+ kws->pending = 1;
+ }
+ local_irq_restore(flags);
}

/**



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