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

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Date:	Wed, 5 Jan 2011 01:49:06 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Zhaolei <zhaolei@...fujitsu.com>,
	"nhorman@...driver.com" <nhorman@...driver.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC patch 2/5] trace event skb fix unassigned field

On Tue, Jan 04, 2011 at 07:40:38PM -0500, Mathieu Desnoyers wrote:
> * Frederic Weisbecker (fweisbec@...il.com) wrote:
> > On Tue, Jan 04, 2011 at 06:46:06PM -0500, nhorman@...driver.com wrote:
> > > Acked- by: Neil Horman <nhorman@...driver.com>
> > > 
> > > 
> > > Sent from my Verizon Wireless Phone
> > > 
> > > ----- Reply message -----
> > > From: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> > > Date: Tue, Jan 4, 2011 6:16 pm
> > > Subject: [RFC patch 2/5] trace event skb fix unassigned field
> > > To: "LKML" <linux-kernel@...r.kernel.org>
> > > Cc: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>, "Steven Rostedt" <rostedt@...dmis.org>, "Frederic Weisbecker" <fweisbec@...il.com>, "Ingo Molnar" <mingo@...e.hu>, "Neil Horman" <nhorman@...driver.com>, "Thomas Gleixner" <tglx@...utronix.de>
> > > 
> > > 
> > > 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?
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