netdev - Re: can we reuse an skb

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Message-Id: <1245677654.4541.37.camel@localhost.localdomain>
Date:	Mon, 22 Jun 2009 19:04:14 +0530
From:	Philby John <pjohn@...mvista.com>
To:	Nicholas Van Orton <turanammo@...il.com>
Cc:	Peter Chacko <peterchacko35@...il.com>, jon_zhou@...lent.com,
	radhamohan_ch@...oo.com, netdev@...r.kernel.org
Subject: Re: can we reuse an skb

On Fri, 2009-06-19 at 15:41 +0530, Nicholas Van Orton wrote:
> Does this mean that when skb buffer has been allocated using
> dev_alloc_skb(), filled with received data and passed to the upper
> layers
> the kernel would automatically do the task of releasing this buffer
> without the user calling dev_kfree_skb()?

Yes, I think that is the case. Except when the user calls an ioctl that
closes your ethernet device, by say using $ifconfig eth0 down, in which
case you must free the ring skb buffer's allocated using
dev_kfree_skb().

>  I once got
> KERNEL: assertion (!atomic_read(&skb->users)) failed at net/core/dev.c
> errors when trying
> to free them using dev_kfree_skb()
> 
> Could this be cause I did not wait until netif_rx_completed() was called?

You are using an old version of the kernel, can't see such code in
2.6.30. From what I know, this usually happens if skb->users is not
equal to one, which means the buffer is in use by some user. Like I
said, you needn't call dev_kfree_skb() explicitly, it will be freed
after use by the upper network layers.

netif_receive_skb() ->deliver_skb()-> pt_prev->func() ->
ip_rcv() -> ip_rcv_finish()

ip_rcv_finish() would finally free it as per the specified protocol.
This I think is the flow, but I guess there would be experts here who
would correct me if I am wrong.

-Philby

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