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Re: tun.c patch to fix "smp_processor_id() in preemptible code"

To: "David S. Miller" <davem@xxxxxxxxxxxxx>
Subject: Re: tun.c patch to fix "smp_processor_id() in preemptible code"
From: Lee Revell <rlrevell@xxxxxxxxxxx>
Date: Tue, 19 Oct 2004 18:51:28 -0400
Cc: herbert@xxxxxxxxxxxxxxxxxxx, vda@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx, Linux Network Development <netdev@xxxxxxxxxxx>, linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>, maxk@xxxxxxxxxxxx, irda-users@xxxxxxxxxxxxxxxxxxxxx
In-reply-to: <20041019154249.6afcaaad.davem@xxxxxxxxxxxxx>
References: <E1CK1e6-0004F3-00@xxxxxxxxxxxxxxxxxxxxxxxx> <1098222676.23367.18.camel@xxxxxxxxxxxxxxxx> <20041019215401.GA16427@xxxxxxxxxxxxxxxxxxx> <1098223857.23367.35.camel@xxxxxxxxxxxxxxxx> <20041019153308.488d34c1.davem@xxxxxxxxxxxxx> <1098225729.23628.2.camel@xxxxxxxxxxxxxxxx> <20041019154249.6afcaaad.davem@xxxxxxxxxxxxx>
Sender: netdev-bounce@xxxxxxxxxxx
On Tue, 2004-10-19 at 18:42, David S. Miller wrote:
> AIUI the only valid reason to use preempt_disable/enable is in
> > the case of per-CPU data.  This is not "real" per-CPU data, it's a
> > performance hack.  Therefore it would be incorrect to add the preemption
> > protection, the fix is not to manually call do_softirq but to let the
> > softirq run by the normal mechanism.
> > 
> > Am I missing something?
> 
> In code paths where netif_rx_ni() is called, there is not a softirq return
> path check, which is why it is checked here.
> 
> Theoretically, if you remove the check, softirq processing can be deferred
> indefinitely.
> 
> What I'm saying, therefore, is that netif_rx_ni() it not just a performance
> hack, it is necessary for correctness as well.
> 

OK, thanks for clarifying.  The correct patch is therefore:

--- include/linux/netdevice.h~  2004-10-19 18:50:18.000000000 -0400
+++ include/linux/netdevice.h   2004-10-19 18:51:01.000000000 -0400
@@ -696,9 +696,11 @@
  */
 static inline int netif_rx_ni(struct sk_buff *skb)
 {
+       preempt_disable();
        int err = netif_rx(skb);
        if (softirq_pending(smp_processor_id()))
                do_softirq();
+       preempt_enable();
        return err;
 }
 
Lee


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