netdev - Re: [PATCH] Fix panic in virtnet

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Message-Id: <20110718035730.16001.77240.sendpatchset@krkumar2.in.ibm.com>
Date:	Mon, 18 Jul 2011 09:27:30 +0530
From:	Krishna Kumar <krkumar2@...ibm.com>
To:	mst@...hat.com
Cc:	netdev@...r.kernel.org, shemminger@...tta.com, davem@...emloft.net,
	Krishna Kumar <krkumar2@...ibm.com>
Subject: Re: [PATCH] Fix panic in virtnet_remove

"Michael S. Tsirkin" <mst@...hat.com> wrote on 07/17/2011 03:42:15 PM:

> > modprobe -r virtio_net panics in free_netdev() as the
> > dev is already freed in the newly introduced virtnet_free
> > (commit 3fa2a1df9094).
> 
> Good catch, thanks!
> 
> > Since virtnet_remove doesn't require
> > dev after unregister,
> 
> I'm not sure that true: vi used just above the line you remove
> is I think the struct virtnet_info that is allocated
> as part of that structure.

You are right, the dev cannot be freed in the destructor.

> > I am removing the free_netdev call
> > in virtnet_remove instead of in virtnet_free (which seems
> > to be the right place to free the dev). Confirmed that
> > the panic is fixed with this patch.
> 
> This might be just because the memory isn't reused.
> Try enabling slab poisoning, I think you'll observe some problems.
> 
> Do we absolutely have to have a destructor?
> Can't we move the per cpu counter free from
> virtnet_free to virtnet_remove, and get rid of
> virtnet_free completely?

I see some other drivers doing that, e.g. xennet_remove:
	...
	free_percpu(info->stats);
	free_netdev(info->netdev);
	...

How about this patch (compile tested only)?

Signed-off-by: Krishna Kumar <krkumar2@...ibm.com>
---
 drivers/net/virtio_net.c |   10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff -ruNp org/drivers/net/virtio_net.c new/drivers/net/virtio_net.c
--- org/drivers/net/virtio_net.c	2011-07-18 09:14:02.000000000 +0530
+++ new/drivers/net/virtio_net.c	2011-07-18 09:16:35.000000000 +0530
@@ -705,14 +705,6 @@ static void virtnet_netpoll(struct net_d
 }
 #endif
 
-static void virtnet_free(struct net_device *dev)
-{
-	struct virtnet_info *vi = netdev_priv(dev);
-
-	free_percpu(vi->stats);
-	free_netdev(dev);
-}
-
 static int virtnet_open(struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
@@ -959,7 +951,6 @@ static int virtnet_probe(struct virtio_d
 	/* Set up network device as normal. */
 	dev->netdev_ops = &virtnet_netdev;
 	dev->features = NETIF_F_HIGHDMA;
-	dev->destructor = virtnet_free;
 
 	SET_ETHTOOL_OPS(dev, &virtnet_ethtool_ops);
 	SET_NETDEV_DEV(dev, &vdev->dev);
@@ -1122,6 +1113,7 @@ static void __devexit virtnet_remove(str
 	while (vi->pages)
 		__free_pages(get_a_page(vi, GFP_KERNEL), 0);
 
+	free_percpu(vi->stats);
 	free_netdev(vi->dev);
 }
 
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