netdev - Re: sk_buff and reference counting netdev pointers

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Message-ID: <CAM_iQpXKsFA5ZZmBvqJaTJ8H88GmoTRmU0HT=FHH8hPrDxX_4Q@mail.gmail.com>
Date:   Mon, 23 Jan 2017 15:45:08 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Joel Cunningham <joel.cunningham@...com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: sk_buff and reference counting netdev pointers

On Mon, Jan 23, 2017 at 2:37 PM, Joel Cunningham <joel.cunningham@...com> wrote:
> 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!
>

This is a very common question.

synchronize_net() is supposed to wait for on-flying packets, since
both for TX and RX paths we acquire RCU read lock.