Re: [PATCH] printk: robustify printk

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Aug 08, 2008 at 08:14:28PM +0200, Peter Zijlstra wrote:
> Avoid deadlocks against rq->lock and xtime_lock by using RCU to defer
> the klogd wakeup.

Longer term, I will have one of the people here (who just happens to have
made the mistake of expressing an interest in hacking on RCU) look into
permitting invocation of RCU earlier during boot, as in before rcu_init()
has been called.  Should be possible, in fact should be possible without
common-case performance penalty.  ;-)

In the meantime, looks good, so:

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> 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);
>  }
> 
>  /**
> 
>