linux-kernel - Re: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog and touch_softlockup

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Date:	Tue, 17 Aug 2010 09:13:20 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Yong Zhang <yong.zhang0@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog
 and touch_softlockup_watchdog

On Tue, Aug 17, 2010 at 01:39:48PM +0300, Sergey Senozhatsky wrote:
> Please kindly review.

I don't have a deep enough understanding of the subtleties between
per_cpu, __get_cpu_var, and __raw_get_cpu_var to really say which is
correct.  To me, all three versions of your patch look they do the same
thing.

Technically, it seems like preempt_disable/enable would be the correct
thing to do.  But as someone pointed out earlier, if the code is preempted
and switches cpu, then the touch_*_watchdog effectively becomes a no-op
(which I guess it can do even with the preempt_disable/enable surrounding
it).  So I have no idea.  I am going to wait for smarter people than me to
provide an opinion. :-)

Cheers,
Don

> 
> ---
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 613bc1f..22dd388 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -116,13 +116,12 @@ static unsigned long get_sample_period(void)
>  static void __touch_watchdog(void)
>  {
>  	int this_cpu = smp_processor_id();
> -
> -	__get_cpu_var(watchdog_touch_ts) = get_timestamp(this_cpu);
> +	per_cpu(watchdog_touch_ts, this_cpu) = get_timestamp(this_cpu);
>  }
>  
>  void touch_softlockup_watchdog(void)
>  {
> -	__get_cpu_var(watchdog_touch_ts) = 0;
> +	__raw_get_cpu_var(watchdog_touch_ts) = 0;
>  }
>  EXPORT_SYMBOL(touch_softlockup_watchdog);
>  
> @@ -142,7 +141,7 @@ void touch_all_softlockup_watchdogs(void)
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>  void touch_nmi_watchdog(void)
>  {
> -	__get_cpu_var(watchdog_nmi_touch) = true;
> +	__raw_get_cpu_var(watchdog_nmi_touch) = true;
>  	touch_softlockup_watchdog();
>  }
>  EXPORT_SYMBOL(touch_nmi_watchdog);
> 
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