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

From: Eduardo Otubo <[email-masked]>
Date: Mon, 20 Nov 2017 13:59:07 +0100

On Mon, Nov 20, 2017 at 12:17:11PM +0100, Juergen Gross wrote:
> On 20/11/17 11:49, Wei Liu wrote:
> > CC netfront maintainers.
> > 
> > On Mon, Nov 20, 2017 at 11:41:09AM +0100, Eduardo Otubo wrote:
> >> 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();
> >> +	}
> 
> I really don't like the busy waits.
> 
> Can't you use e.g. a wait queue and wait_event_interruptible() instead?

I thought about using these, but I don't think the busy waits here are much of a
problem because it's just unloading a kernel module, not a very repetitive
action. But yes I can go for this approach on v2.

> 
> BTW: what happens if the device is already in closed state if you enter
> xennet_remove()? In case this is impossible, please add a comment to
> indicate you've thought about that case.

Looks like this is the same problem Paul Durrant mentioned on his comment. I'll
work on this as well on v2.

Thanks for the review and the help on IRC :-)