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Date: Mon, 6 Nov 2017 11:40:38 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: James Morse <james.morse@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: get_online_cpus() from a  preemptible() context (bug?)
Message-ID: <20171106104038.GD3857@worktop>
References: <59FC8119.8030608@arm.com>
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On Mon, Nov 06, 2017 at 11:32:12AM +0100, Peter Zijlstra wrote:
> On Fri, Nov 03, 2017 at 02:45:45PM +0000, James Morse wrote:
> > Hi Thomas, Peter,
> > 
> > I'm trying to work out what stops a thread being pre-empted and migrated between
> > calling get_online_cpus() and put_online_cpus().

Nothing; why would you think it would? All those functions guarantee is
that any CPU observed as being online says online (and its converse,
that a CPU observed as being offline, says offline, although less people
care about that one).

That is; it serializes against CPU hotplug, nothing else.