netdev - Re: BUG: soft lockup detected on CPU#0! (2.6.18.2 plus hacks)

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Date:	Tue, 9 Jan 2007 09:10:45 +0100
From:	Jarek Poplawski <jarkao2@...pl>
To:	Stephen Hemminger <shemminger@...l.org>
Cc:	Ben Greear <greearb@...delatech.com>,
	David Miller <davem@...emloft.net>,
	herbert@...dor.apana.org.au, dlstevens@...ibm.com,
	netdev@...r.kernel.org
Subject: Re: BUG: soft lockup detected on CPU#0! (2.6.18.2 plus hacks)

On Mon, Jan 08, 2007 at 10:03:50AM -0800, Stephen Hemminger wrote:
> On Mon, 08 Jan 2007 08:57:10 -0800
> Ben Greear <greearb@...delatech.com> wrote:
> 
> > Jarek Poplawski wrote:
> > > On Fri, Jan 05, 2007 at 12:33:43PM -0800, Ben Greear wrote:
> > > ...
> > >   
> > >> So, I do believe this was the problem we were hitting, and it seems fixed.
> > >>     
> > >
> > > Congratulations!
> > >
> > > But I can see one strange thing in vlan.c:
> > >
> > > /* Must be invoked with RCU read lock (no preempt) */
> > > static struct vlan_group *__vlan_find_group(int real_dev_ifindex)
> > > ...
> > >  * Must be invoked with RCU read lock (no preempt)
> > >  */
> > > struct net_device *__find_vlan_dev(struct net_device *real_dev,
> > > ...
> > >
> > > But later in this file no sign of disabling preemption
> > > for these calls and for hlist_add_head_rcu and hlist_del_rcu.
> > >
> > > I can't imagine how this works?
> 
> Preempt is already disabled on the receive path.

I'm not sure you're talking about the same thing -
there is blocking possible inside register_vlan_dev
and unregister_vlan_dev, grp pointer is held during
this blocking - I've thought it's only possible in
sleepable RCU...

Jarek P.
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