Re: [RFC patch 2/5] trace event skb fix unassigned field

[Neil Horman]

On Tue, Jan 04, 2011 at 08:21:23PM -0500, Mathieu Desnoyers wrote:
> * Frederic Weisbecker (fweisbec@xxxxxxxxx) wrote:
> > On Tue, Jan 04, 2011 at 07:40:38PM -0500, Mathieu Desnoyers wrote:
> > > * Frederic Weisbecker (fweisbec@xxxxxxxxx) wrote:
> > > > On Tue, Jan 04, 2011 at 06:46:06PM -0500, nhorman@xxxxxxxxxxxxx wrote:
> > > > > Acked- by: Neil Horman <nhorman@xxxxxxxxxxxxx>
> > > > > 
> > > > > 
> > > > > Sent from my Verizon Wireless Phone
> > > > > 
> > > > > ----- Reply message -----
> > > > > From: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> > > > > Date: Tue, Jan 4, 2011 6:16 pm
> > > > > Subject: [RFC patch 2/5] trace event skb fix unassigned field
> > > > > To: "LKML" <linux-kernel@xxxxxxxxxxxxxxx>
> > > > > Cc: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>, "Steven Rostedt" <rostedt@xxxxxxxxxxx>, "Frederic Weisbecker" <fweisbec@xxxxxxxxx>, "Ingo Molnar" <mingo@xxxxxxx>, "Neil Horman" <nhorman@xxxxxxxxxxxxx>, "Thomas Gleixner" <tglx@xxxxxxxxxxxxx>
> > > > > 
> > > > > 
> > > > > The field "protocol" in event kfree_skb is left unassigned if skb is NULL,
> > > > > leaving its trace output as garbage. Assign the value to 0 when skb is NULL
> > > > > instead.
> > > > 
> > > > Hm, if the skb is already null, we probably shouldn't send any trace.
> > > > 
> > > > What about using TP_CONDITION() ?
> > > 
> > > Hrm, let's see. It's been introduced by commit
> > > 5cb3d1d9d34ac04bcaa2034139345b2a5fea54c1
> > > by Zhaolei.
> > > 
> > > Event at the time of that commit, the only caller looked like:
> > > 
> > > void kfree_skb(struct sk_buff *skb)
> > > {
> > >         if (unlikely(!skb))
> > >                 return;
> > >         if (likely(atomic_read(&skb->users) == 1))
> > >                 smp_rmb();
> > >         else if (likely(!atomic_dec_and_test(&skb->users)))
> > >                 return;
> > >         trace_kfree_skb(skb, __builtin_return_address(0));
> > >         __kfree_skb(skb);
> > > }
> > > EXPORT_SYMBOL(kfree_skb);
> > > 
> > > So it already checks for a null pointer before calling the tracepoint. This
> > > leads me to wonder why why this check was added in the first place ?
> > 
> > Likely for no strong reasons :)
> > 
> > So I guess we can remove the check from the tracepoint?
> 
> Yep, leading to this patch instead:
> 
> trace event skb remove duplicate null-pointer check
> 
> The check for NULL skb in the kfree_skb trace event is a duplicate from the
> check already done in its only caller, kfree_skb(). Remove this duplicate check.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Neil Horman <nhorman@xxxxxxxxxxxxx>
> ---
>  include/trace/events/skb.h |    4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> Index: linux-2.6-lttng/include/trace/events/skb.h
> ===================================================================
> --- linux-2.6-lttng.orig/include/trace/events/skb.h
> +++ linux-2.6-lttng/include/trace/events/skb.h
> @@ -25,9 +25,7 @@ TRACE_EVENT(kfree_skb,
>  
>  	TP_fast_assign(
>  		__entry->skbaddr = skb;
> -		if (skb) {
> -			__entry->protocol = ntohs(skb->protocol);
> -		}
> +		__entry->protocol = ntohs(skb->protocol);
>  		__entry->location = location;
>  	),
>  
> 
> 
Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>

> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
> 
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