linux-kernel - Re: [PATCH v2] xen: avoid crash in disable_hotplug

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Date:   Thu, 6 Sep 2018 22:31:45 +0200
From:   Olaf Hering <olaf@...fle.de>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     xen-devel@...ts.xenproject.org, Juergen Gross <jgross@...e.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] xen: avoid crash in disable_hotplug_cpu

Am Thu, 6 Sep 2018 14:45:57 -0400
schrieb Boris Ostrovsky <boris.ostrovsky@...cle.com>:

> On 09/06/2018 02:37 AM, Olaf Hering wrote:
> > The command 'xl vcpu-set 0 0', issued in dom0, will crash dom0:

> > This happens because handle_vcpu_hotplug_event is called twice. In the
> > first iteration cpu_present is still true, in the second iteration
> > cpu_present is false which causes get_cpu_device to return NULL.
> > In case of cpu#0, cpu_online is apparently always true.

> I think we should check both this and num_online_cpus() != 0.

This can not possibly help. cpu#0 is the first one that goes offline.
IF cpu0_hotpluggable is broken, then only "if (!cpu) return;" can help.

Olaf

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