linux-kernel - Re: [patch 1/3] printk: fix wake_up

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Date:	Fri, 26 Nov 2010 13:10:08 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org,
	Christof Schmitt <christof.schmitt@...ibm.com>,
	Frank Blaschka <frank.blaschka@...ibm.com>,
	Horst Hartmann <horsth@...ux.vnet.ibm.com>
Subject: Re: [patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug

On Fri, 2010-11-26 at 13:00 +0100, Heiko Carstens wrote:
> plain text document attachment (001_printk_preempt.diff)
> From: Heiko Carstens <heiko.carstens@...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 get_cpu_var() instead which disables preemption and makes sure that
> the outlined scenario cannot happen.
> 
> Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
> ---
>  kernel/printk.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1087,8 +1087,10 @@ int printk_needs_cpu(int cpu)
>  
>  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);
> +	}
>  }
>  
>  /**
> 

But but but, the cpu can still be offlined between writing this state
and the next tick happening, right?
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