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

[Mathieu Desnoyers]

* 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;
 	),
 


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