lkml.org 
[lkml]   [2004]   [Oct]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: tun.c patch to fix "smp_processor_id() in preemptible code"
On Tue, 19 Oct 2004 18:42:11 -0400
Lee Revell <rlrevell@joe-job.com> wrote:

> On Tue, 2004-10-19 at 18:33, David S. Miller wrote:
> > On Tue, 19 Oct 2004 18:10:58 -0400
> > Lee Revell <rlrevell@joe-job.com> 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?

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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 14:07    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site