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SubjectRe: [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@google.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

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