linux-kernel - Re: get_online_cpus() from a preemptible() context (bug?)

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Message-ID: <20171106104038.GD3857@worktop>
Date:   Mon, 6 Nov 2017 11:40:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     James Morse <james.morse@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: get_online_cpus() from a  preemptible() context (bug?)

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.