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Subject: Re: [PATCH v2] xen: avoid crash in disable_hotplug_cpu
To:     Olaf Hering <olaf@aepfle.de>
Cc:     xen-devel@lists.xenproject.org, Juergen Gross <jgross@suse.com>,
        open list <linux-kernel@vger.kernel.org>
References: <20180906063712.28639-1-olaf@aepfle.de>
 <b7306d7f-e827-6556-e1bb-4b934a162e46@oracle.com>
 <20180906223145.37f7009b.olaf@aepfle.de>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <904b8bf8-ce80-2751-04bf-de1e450ef58c@oracle.com>
Date:   Thu, 6 Sep 2018 18:53:40 -0400
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From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: Olaf Hering <olaf@aepfle.de>
Cc: xen-devel@lists.xenproject.org, Juergen Gross <jgross@suse.com>,
 open list <linux-kernel@vger.kernel.org>
Message-ID: <904b8bf8-ce80-2751-04bf-de1e450ef58c@oracle.com>
Subject: Re: [PATCH v2] xen: avoid crash in disable_hotplug_cpu
References: <20180906063712.28639-1-olaf@aepfle.de>
 <b7306d7f-e827-6556-e1bb-4b934a162e46@oracle.com>
 <20180906223145.37f7009b.olaf@aepfle.de>
In-Reply-To: <20180906223145.37f7009b.olaf@aepfle.de>

--Qsz2hA954rg39Vg9lPeOJz7WCrKPmXf3t
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On 09/06/2018 04:31 PM, Olaf Hering wrote:
> Am Thu, 6 Sep 2018 14:45:57 -0400
> schrieb Boris Ostrovsky <boris.ostrovsky@oracle.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 th=
e
>>> 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() !=3D 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.=


And maybe that needs to be part of the check, in addition to
cpu_is_hotpluggable() test.

Offlining CPU0 is problematic. For example, look at xen_pv_cpu_disable().=


-boris


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