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Subject: Re: [patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        linux-kernel@vger.kernel.org,
        Christof Schmitt <christof.schmitt@de.ibm.com>,
        Frank Blaschka <frank.blaschka@de.ibm.com>,
        Horst Hartmann <horsth@linux.vnet.ibm.com>
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Date: Fri, 26 Nov 2010 14:01:52 +0100
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Le vendredi 26 novembre 2010 à 13:42 +0100, Heiko Carstens a écrit :
> On Fri, Nov 26, 2010 at 01:35:09PM +0100, Eric Dumazet wrote:
> > Le vendredi 26 novembre 2010 à 13:00 +0100, Heiko Carstens a écrit :
> > >  void wake_up_klogd(void)
> > >  {
> > > -	if (waitqueue_active(&log_wait))
> > > -		__raw_get_cpu_var(printk_pending) = 1;
> > > +	if (waitqueue_active(&log_wait)) {
> > > +		get_cpu_var(printk_pending) = 1;
> > > +		put_cpu_var(printk_pending);
> > > +	}
> > >  }
> > 
> > Please use :
> > 
> > this_cpu_write(printk_pending, 1);
> > 
> > It is faster on x86, and does the right thing too.
> 
> Ah, right. I wasn't aware that such a thing even exists.
> Updated patch below:
> 
> Subject: [PATCH] printk: fix wake_up_klogd() vs cpu hotplug
> 
> From: Heiko Carstens <heiko.carstens@de.ibm.com>
> 
> wake_up_klogd() may get called from preemtible context but uses
> __raw_get_cpu_var() to write to a per cpu variable. If it gets preempted between
> getting the address and writing to it, the cpu in question could be offline if
> the process gets scheduled back and hence writes to the per cpu data of an offline
> cpu.
> 
> No idea why that behaviour was introduced with fa33507a "printk: robustify
> printk, fix #2" which was supposed to fix a "using smp_processor_id() in
> preemptible" warning.
> 
> Let's use this_cpu_write() instead which disables preemption and makes sure that
> the outlined scenario cannot happen.
> 


> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> ---
>  kernel/printk.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1088,7 +1088,7 @@ int printk_needs_cpu(int cpu)
>  void wake_up_klogd(void)
>  {
>  	if (waitqueue_active(&log_wait))
> -		__raw_get_cpu_var(printk_pending) = 1;
> +		this_cpu_write(printk_pending, 1);
>  }
>  
>  /**

Acked-by: Eric Dumazet <eric.dumazet@gmail.com>