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

From: Lee Revell <[email-masked]>
Date: Tue, 19 Oct 2004 18:42:11 -0400

On Tue, 2004-10-19 at 18:33, David S. Miller wrote:
> On Tue, 19 Oct 2004 18:10:58 -0400
> Lee Revell <[email-masked]> wrote:
> 
> >   /*
> >    * Since receiving is always initiated from a tasklet (in iucv.c),
> >    * we must use netif_rx_ni() instead of netif_rx()
> >    */
> > 
> > This implies that the author thought it was a matter of correctness to
> > use netif_rx_ni vs. netif_rx.  But it looks like the only difference is
> > that the former sacrifices preempt-safety for performance.
> 
> You can't really delete netif_rx_ni(), so if there is a preemptability
> issue, just add the necessary preemption protection around the softirq
> checks.
> 

Why not?  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?

Lee