netdev - Re: pktgen question

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Message-ID: <46F6A887.1030301@opengridcomputing.com>
Date:	Sun, 23 Sep 2007 12:55:19 -0500
From:	Steve Wise <swise@...ngridcomputing.com>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
CC:	netdev@...r.kernel.org
Subject: Re: pktgen question



Evgeniy Polyakov wrote:
> Hi Steve.
> 
> On Sun, Sep 23, 2007 at 11:12:12AM -0500, Steve Wise (swise@...ngridcomputing.com) wrote:
>> The pktgen module provides a way to "clone" the skb its using for 
>> transmission, and allows passing N clones of the originally created skb 
>> to the driver under test.    However, it doesn't really use skb_clone(), 
>> but rather it just bumps the skb->users count for each "clone" and 
>> passes the same skb ptr to the driver.
>>
>> Q: Is that a valid use of skb->users or should pktgen really be cloning 
>> the skbuff?
> 
> It's a hack, but since skb is owned by pktgen only (no copies in some
> outside queues or some other access) it is allowed just to bump reference
> counter (i.e. 'share' skb in usual notation).
>

Its a hack that breaks cxgb3 because cxgb3 uses the skb->cb area for 
each skb passed down.  So cxgb3 is at fault then?  IE a driver cannot 
use the skb->cb field if the users count is > 1?  Or maybe a driver can 
_never_ use the cb field?

Steve.




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