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