netdev - Re: [PATCH] skb: Augment skb_copy_datagram_iovec TRACE

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Message-ID: <alpine.DEB.2.00.0908311503170.24907@gandalf.stny.rr.com>
Date:	Mon, 31 Aug 2009 15:05:44 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Neil Horman <nhorman@...driver.com>
cc:	netdev@...r.kernel.org, davem@...emloft.net, mingo@...e.hu
Subject: Re: [PATCH] skb: Augment skb_copy_datagram_iovec TRACE_EVENT to dump
 more info

On Fri, 28 Aug 2009, Neil Horman wrote:
> > >  
> > >  /*
> > >   * Tracepoint for free an sk_buff:
> > > @@ -42,16 +43,35 @@ TRACE_EVENT(skb_copy_datagram_iovec,
> > >  	TP_ARGS(skb, len),
> > >  
> > >  	TP_STRUCT__entry(
> > > -		__field(	const void *,		skbaddr		)
> > > +		__field(	const struct sk_buff *,	skb		)
> > >  		__field(	int,			len		)
> > > +		__field(	int,			anid		)
> > > +		__field(	int,			cnid		)
> > > +		__field(	int,			rx_queue	)
> > > +		__dynamic_array(char,		name,	IFNAMSIZ	)
> > 
> > For string fields we have a __string macro:
> > 
> > 		__string(	char,			name		)
> > 
> Yeah, I was going to use that, but I ran into an issue, in that the __string
> macro expects, as its second of two arguments, a pointer to a string with witch
> to determine the dynamic arrays size.  The problem is that, if I'm to provide
> that, I need to point it to skb->dev->name, and there is no guarantee that
> skb->dev isn't NULL, at which point I think the use of string breaks down.  Note
> below that I obtain the dev pointer by other means and check its validity prior
> to assigning it.  If theres a better way to do this, please let me know.

Ah, OK I see the issue. The problem is the code that figures out the 
offset. It will always be calculated, and that would cause the skb->dev be 
evaluated and if NULL will cause an OOPS.

Yeah, keep the dynamic_array here then.

-- Steve

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