netdev - Re: 2.6.23 WARNING: at kernel/softirq.c:139 local_bh

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Date:	Fri, 23 Nov 2007 20:57:57 +0300
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Simon Arlott <simon@...e.lp0.eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: 2.6.23 WARNING: at kernel/softirq.c:139 local_bh_enable()

On Fri, Nov 23, 2007 at 11:07:56AM -0600, Matt Mackall (mpm@...enic.com) wrote:
> On Fri, Nov 23, 2007 at 01:55:19PM +0300, Evgeniy Polyakov wrote:
> > On Fri, Nov 23, 2007 at 12:21:57AM -0800, Andrew Morton (akpm@...ux-foundation.org) wrote:
> > > > [2059664.615816] __iptables__: init4 IN=ppp0 OUT=ppp0 WARNING: at kernel/softirq.c:139 local_bh_enable()
> > > > [2059664.620535]  [<80120364>] local_bh_enable+0x3c/0x97
> > 
> > > > [2059664.620657]  [<8011c205>] __call_console_drivers+0x61/0x6d
> > > > [2059664.620669]  [<8011c3fc>] release_console_sem+0x164/0x1bf
> > > > [2059664.620679]  [<8011c81f>] vprintk+0x27a/0x2ff
> >  
> > > If that trace is to be beieved we're doing nefilter stuff on packets which
> > > were sent across netconsole.
> > > 
> > > This probably isn't anything the netfilter guys have thought about.  And
> > > probably we don't want them to.  Is there some simple way in which we can
> > > exempt netconsole from netfilter processing?
> > 
> > This is not about netfilter, but about freeing skb in interrupt context, 
> > which is not allowed, and in interrupt skbs are queued to be freed in softirq,
> > but netcnsole wants to flush softirq freeing queue. That is a question: why?
> 
> My memory here is hazy, but I think this exists to rescue netconsole
> in low-memory situations. This bit originated with Ingo, so maybe he
> can recall.
> 
> Netpoll can process an arbitrary number of skbs inside a single
> interrupt. Think sysrq-t at one packet per line or kgdboe where the
> entire trace session can happen inside one very long interrupt.
> 
> Perhaps we can refine this to mark netpoll's skbs (perhaps with
> ->destructor?) and delete only skbs we own. As these are never passed
> through any of the other route/xfrm/filter code, they should be safe
> to delete even in irq context, yes?
> 
> > Removing zap_completion_queue() from find_skb() will fix the warning,
> > but I'm not sure this is a correct fix. I've added Matt to the Cc list.
> 
> Care to try the sysrq-t or OOM message tests?

We basically can not free skbs there - if it is interrupt context and
we are freeing some skb with destructor we will catch the warning anyway.

No matter if we are under memory pressure or whatever - it is not
allowed - a lot of skbs are supposed to be freed in softirq context,
that is why dev_kfree_skb_any() exists.

I think we can drop skbs _without_ destructor from the queue though in
that conditions given that we actually need only one.

-- 
	Evgeniy Polyakov
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