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

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Date:	Mon, 16 Aug 2010 22:06:49 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: fix BUG: using smp_processor_id() in touch_nmi_watchdog and
 touch_softlockup_watchdog

On Mon, Aug 16, 2010 at 09:34:52AM -0400, Don Zickus wrote:
> I don't recall any requirement to have preemption disabled when using
> those functions.

Isn't that implicit? I mean the caller of touch_{softlockup|nmi}_watchdog
will sticky to that cpu before it finish running.

> It seems sensible to put it in the
> touch_{softlockup|nmi}_watchdog code.

I don't think so. Such as:

	...
      preempt_disable()                <===A
      touch_{softlockup|nmi}_watchdog  <===B
      preempt_enable()                 <===C
        ...

You just scroll A and C into B, but what will happen before preempt
occur before A?

> 
> I assume the reason for having preemption disabled when using
> smp_processor_id() is that the code could migrate to another cpu when
> rescheduled?

If the migration could happen, then we could touch the wrong cpu-data,
and the detection on the original cpu will trigger anyway.

> 
> I don't see a problem with the patch, but my low level understanding of
> the __get_cpu_var vs. per_cpu isn't very strong.

Maybe we should use __raw_get_cpu_var() instead.

Thanks,
Yong
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