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

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Message-ID: <20131126164552.GF2959@phenom.dumpdata.com>
Date:	Tue, 26 Nov 2013 11:45:52 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Vrabel <david.vrabel@...rix.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 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.

> 
> David
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