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Subject: Re: [PATCH 1/4] cpu_stop: implement stop_cpu[s]()
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
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Date: Tue, 04 May 2010 10:43:40 +0200
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On Tue, 2010-05-04 at 09:03 +0200, Tejun Heo wrote:
> On 05/04/2010 08:36 AM, Tejun Heo wrote:
> > On 05/03/2010 03:26 PM, Peter Zijlstra wrote:
> >> On Thu, 2010-04-22 at 18:09 +0200, Tejun Heo wrote:
> >>> +static int cpu_stopper_thread(void *data)
> >>> +{
> >>> +       struct cpu_stopper *stopper = data; 
> >>
> >> BUG_ON(stopper != __get_cpu_var(cpu_stopper)); ?
> > 
> > Added.
> 
> Now that I think more about it, there's a subtle race condition with
> the above BUG_ON().  Stoppers are prepared by CPU_UP_PREPARE and
> started by CPU_ONLINE but brought down by CPU_DEAD.  IOW, they're
> allowed to run detached from their designated CPUs between CPU_DYING
> and CPU_DEAD (the reponsibility of guaranteeing target cpus's onliness
> is on the callers).  So, the above BUG_ON() might trigger spuriously
> if a cpu goes down after brought online before its cpu_stopper had a
> chance to pass through the BUG_ON() test.

Ah indeed. A well, drop it then, its not worth making a more complicated
test.