Subject: Re: [PATCH] xen-netfront: remove warning when unloading module

From: 'Eduardo Otubo' <[email-masked]>
Date: Mon, 20 Nov 2017 13:56:10 +0100

On Mon, Nov 20, 2017 at 10:55:55AM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Eduardo Otubo [mailto:[email-masked]]
> > Sent: 20 November 2017 10:41
> > To: [email-masked]
> > Cc: [email-masked]; Paul Durrant <[email-masked]>; Wei
> > Liu <[email-masked]>; [email-masked];
> > [email-masked]; [email-masked]; [email-masked];
> > [email-masked]; Eduardo Otubo <[email-masked]>
> > Subject: [PATCH] xen-netfront: remove warning when unloading module
> > 
> > When unloading module xen_netfront from guest, dmesg would output
> > warning messages like below:
> > 
> >   [  105.236836] xen:grant_table: WARNING: g.e. 0x903 still in use!
> >   [  105.236839] deferring g.e. 0x903 (pfn 0x35805)
> > 
> > This problem relies on netfront and netback being out of sync. By the time
> > netfront revokes the g.e.'s netback didn't have enough time to free all of
> > them, hence displaying the warnings on dmesg.
> > 
> > The trick here is to make netfront to wait until netback frees all the g.e.'s
> > and only then continue to cleanup for the module removal, and this is done
> > by
> > manipulating both device states.
> > 
> > Signed-off-by: Eduardo Otubo <[email-masked]>
> > ---
> >  drivers/net/xen-netfront.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> > index 8b8689c6d887..b948e2a1ce40 100644
> > --- a/drivers/net/xen-netfront.c
> > +++ b/drivers/net/xen-netfront.c
> > @@ -2130,6 +2130,17 @@ static int xennet_remove(struct xenbus_device
> > *dev)
> > 
> >  	dev_dbg(&dev->dev, "%s\n", dev->nodename);
> > 
> > +	xenbus_switch_state(dev, XenbusStateClosing);
> > +	while (xenbus_read_driver_state(dev->otherend) !=
> > XenbusStateClosing){
> > +		cpu_relax();
> > +		schedule();
> > +	}
> > +	xenbus_switch_state(dev, XenbusStateClosed);
> > +	while (dev->xenbus_state != XenbusStateClosed){
> > +		cpu_relax();
> > +		schedule();
> > +	}
> > +
> 
> Waitiing for closing should be ok but waiting for closed is risky. As soon as a backend is in the closed state then a toolstack can completely remove the backend xenstore area, resulting a state of XenbusStateUnknown, which would cause your second loop to spin forever.
> 
>   Paul

Well, that's a scenario I didn't foresee. I'll come up with a solution in order
avoid this problem. Thanks for the review.

> 
> >  	xennet_disconnect_backend(info);
> > 
> >  	unregister_netdev(info->netdev);
> > --
> > 2.13.6
> 

-- 
Eduardo Otubo