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

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Message-ID: <1281968948.1926.1467.camel@laptop>
Date:	Mon, 16 Aug 2010 16:29:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	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, 2010-08-16 at 10:12 -0400, Don Zickus wrote:
> On Mon, Aug 16, 2010 at 03:46:58PM +0200, Peter Zijlstra wrote:
> > > 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.
> > 
> > __get_cpu_var() gets you the value on the current cpu, per_cpu() takes a
> > cpu argument.
> 
> Well I know that much. :-)  It seems that __get_cpu_var depends on
> preemption being disabled whereas per_cpu does not?  Though for some
> reason I thought __get_cpu_var would be more atomic when it grabbed the
> current cpu such that you wouldn't need to disable preemption.  Guess not.

Indeed, it can't be implemented atomically on all smp systems, hence its
really nothing other than a 'convenient' short for per_cpu(foo,
smp_processor_id()).
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