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

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Message-ID: <20171120125610.GA12153@vader>
Date:   Mon, 20 Nov 2017 13:56:10 +0100
From:   'Eduardo Otubo' <otubo@...hat.com>
To:     Paul Durrant <Paul.Durrant@...rix.com>
Cc:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Wei Liu <wei.liu2@...rix.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "cavery@...hat.com" <cavery@...hat.com>,
        "cheshi@...hat.com" <cheshi@...hat.com>,
        "mgamal@...hat.com" <mgamal@...hat.com>
Subject: Re: [PATCH] xen-netfront: remove warning when unloading module

On Mon, Nov 20, 2017 at 10:55:55AM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Eduardo Otubo [mailto:otubo@...hat.com]
> > Sent: 20 November 2017 10:41
> > To: xen-devel@...ts.xenproject.org
> > Cc: netdev@...r.kernel.org; Paul Durrant <Paul.Durrant@...rix.com>; Wei
> > Liu <wei.liu2@...rix.com>; linux-kernel@...r.kernel.org;
> > vkuznets@...hat.com; cavery@...hat.com; cheshi@...hat.com;
> > mgamal@...hat.com; Eduardo Otubo <otubo@...hat.com>
> > 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 <otubo@...hat.com>
> > ---
> >  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