linux-kernel - Re: [PATCH 3/4] xen/manage: Guard against user-space initiated poweroff and XenBus.

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Message-ID: <529C6EAC.4030408@citrix.com>
Date:	Mon, 2 Dec 2013 11:27:40 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<Ian.Campbell@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<linux-kernel@...r.kernel.org>, <JBeulich@...e.com>,
	<boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 3/4] xen/manage: Guard against user-space initiated poweroff
 and XenBus.

On 26/11/13 16:45, Konrad Rzeszutek Wilk wrote:
> On Thu, Nov 21, 2013 at 11:09:52AM +0000, David Vrabel wrote:
>> On 08/11/13 17:38, Konrad Rzeszutek Wilk wrote:
>>> There is a race case where the user does 'poweroff'
>>> and at the same time the system admin does 'xl shutdown'.
>>
>> This isn't a Xen-specific problem is it?  Wouldn't it be better to fix
>> this in generic code?
> 
> Possibly. I believe the reason for the reboot_notifier to exist is
> to provide a means to fix the race.
> 
>>
>> Especially since I don't think this patch actually fixes the race
>> completely.
>>
>>> --- a/drivers/xen/manage.c
>>> +++ b/drivers/xen/manage.c
>> [...]
>>> @@ -222,7 +230,7 @@ static void shutdown_handler(struct xenbus_watch *watch,
>>>  	};
>>>  	static struct shutdown_handler *handler;
>>>  
>>> -	if (shutting_down != SHUTDOWN_INVALID)
>>> +	if (atomic_read(&shutting_down) != SHUTDOWN_INVALID)
>>>  		return;
>>
>> In guest initiated poweroff at this time will still race with this
>> toolstack initiated poweroff.
> 
> No, b/c the reboot notifier would have set 'shutting_down' already.

If the guest initiated power off is started here, the reboot notifier
won't have run yet.

David
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