linux-kernel - Re: [PATCH v2] netpoll: Fix extra refcount release in netpoll

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Message-ID: <20160325164639.GA29822@localhost>
Date:	Fri, 25 Mar 2016 11:46:39 -0500
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Neil Horman <nhorman@...hat.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
	netdev@...r.kernel.org, Alexander Duyck <aduyck@...antis.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] netpoll: Fix extra refcount release in
 netpoll_cleanup()

On Fri, Mar 25, 2016 at 07:33:42AM -0400, Neil Horman wrote:
> On Thu, Mar 24, 2016 at 09:56:21PM -0500, Bjorn Helgaas wrote:
> > netpoll_setup() does a dev_hold() on np->dev, the netpoll device.  If it
> > fails, it correctly does a dev_put() but leaves np->dev set.  If we call
> > netpoll_cleanup() after the failure, np->dev is still set so we do another
> > dev_put(), which decrements the refcount an extra time.
> > 
> > It's questionable to call netpoll_cleanup() after netpoll_setup() fails,
> > but it can be difficult to find the problem, and we can easily avoid it in
> > this case.  The extra decrements can lead to hangs like this:
> > 
> >   unregister_netdevice: waiting for bond0 to become free. Usage count = -3
> > 
> > Set and clear np->dev at the points where we dev_hold() and dev_put() the
> > device.
> > 
> > Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> > ---
> >  net/core/netpoll.c |    3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> > index 94acfc8..a57bd17 100644
> > --- a/net/core/netpoll.c
> > +++ b/net/core/netpoll.c
> > @@ -603,7 +603,6 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
> >  	const struct net_device_ops *ops;
> >  	int err;
> >  
> > -	np->dev = ndev;
> >  	strlcpy(np->dev_name, ndev->name, IFNAMSIZ);
> >  	INIT_WORK(&np->cleanup_work, netpoll_async_cleanup);
> >  
> > @@ -670,6 +669,7 @@ int netpoll_setup(struct netpoll *np)
> >  		goto unlock;
> >  	}
> >  	dev_hold(ndev);
> > +	np->dev = ndev;
> >  
> >  	if (netdev_master_upper_dev_get(ndev)) {
> >  		np_err(np, "%s is a slave device, aborting\n", np->dev_name);
> > @@ -770,6 +770,7 @@ int netpoll_setup(struct netpoll *np)
> >  	return 0;
> >  
> >  put:
> > +	np->dev = NULL;
> >  	dev_put(ndev);
> >  unlock:
> >  	rtnl_unlock();
> > 
> 
> Is this safe for stacked devices?  It makes good sense for the typical case, but
> if you attempt to setup a netpoll client on a bridge/bond/vlan, etc, the lower
> device will get its own netpoll struct registered and have no associated np->dev
> pointer.  It not be a real problem in practice, But you probably want to check
> to make sure that stacked  devices which recursively call the netpoll api don't
> do anyting with the np->dev pointer.

You're right, there is an issue here.  I reproduced a problem with a
bond device.  bond_netpoll_setup() calls __netpoll_setup() directly
(not netpoll_setup()).  I'll debug it more; just wanted to let you
know there *is* a problem with this patch.

Bjorn