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Date: Wed, 30 Nov 2011 18:13:18 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Dimitri Sivanich <sivanich@sgi.com>
Cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode
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On Wed, 30 Nov 2011 20:07:27 -0600 Dimitri Sivanich <sivanich@sgi.com> wrote:

> > And the whole thing is racy, isn't it?  The "new" CPU can go offline a
> > nanosecond after we performed that test, so why perform it at all?
> 
> See my email concerning the panic in cpu_online().

That doesn't address my question.

What's the point in checking cpu_online() when we have no locks to
prevent the online map from changing?

What happens if this cpu goes offline immediately after that check has
passed?