netdev - sk_buff and reference counting netdev pointers

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Date:   Mon, 23 Jan 2017 16:37:06 -0600
From:   Joel Cunningham <joel.cunningham@...com>
To:     netdev@...r.kernel.org
Subject: sk_buff and reference counting netdev pointers

Hi,

I’m working on a research effort to understand the synchronization mechanisms for accessing and modifying a struct net_device object.  One area that isn’t clear is the net device pointer (dev) stored in a struct sk_buff.  From my investigation, the pointer appears to be assigned without increasing the struct net_device’s reference count (example __netdev_alloc_skb doesn’t call dev_hold) and also when the sk_buff is freed (kfree_skb) no call to dev_put() is made.  This seems to leave a possibility of an skb referencing a stale net device unless something is cleaning up all the skbs during unregister_netdevice() (which waits for all outstanding references to be released).  Any insight in understanding how this is working would be appreciated!

Joel