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

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Date:	Thu, 24 Mar 2016 11:13:34 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
	netdev@...r.kernel.org, Neil Horman <nhorman@...hat.com>,
	Alexander Duyck <aduyck@...antis.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] netpoll: Fix extra refcount release in netpoll_cleanup()

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

In __netpoll_setup(), don't set np->dev until we know we're going to
succeed.

Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
---
 net/core/netpoll.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 94acfc8..32e373e 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);

@@ -628,7 +627,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)

 		atomic_set(&npinfo->refcnt, 1);

-		ops = np->dev->netdev_ops;
+		ops = ndev->netdev_ops;
 		if (ops->ndo_netpoll_setup) {
 			err = ops->ndo_netpoll_setup(ndev, npinfo);
 			if (err)
@@ -639,6 +638,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 		atomic_inc(&npinfo->refcnt);
 	}

+	np->dev = ndev;
 	npinfo->netpoll = np;

 	/* last thing to do is link it to the net device structure */