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Subject: Re: [PATCH 2/4] sched: implement __set_cpus_allowed()
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: mingo@elte.hu, linux-kernel@vger.kernel.org,
       Rusty Russell <rusty@rustcorp.com.au>, Mike Galbraith <efault@gmx.de>
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Date: Mon, 31 May 2010 12:02:45 +0200
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On Mon, 2010-05-31 at 12:01 +0200, Peter Zijlstra wrote:
> >  but we can update cpu_active mask before other
> > CPU_UP_PREPARE notifiers are executed so that it's symmetrical to cpu
> > down path and then this problem goes away the exact same way, right?
> 
> Ah, no, we cannot mark it active before its actually up, because at that
> time we'll actually try and run stuff on it, which clearly won't work
> when its not there to run stuff. 

So we should clear it _before_ we go down, and set it _after_ we're up.