netdev - Re: kernel BUG in ipmr_queue

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Date:	Fri, 30 Oct 2015 11:40:24 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Florian Westphal <fw@...len.de>,
	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ani Sinha <ani@...sta.com>, netdev@...r.kernel.org,
	ani@...rban.org, fruggeri@...sta.com
Subject: Re: kernel BUG in ipmr_queue_xmit()

On Fri, Oct 30, 2015, at 11:36, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > Signed-off-by: Ani Sinha <ani@...sta.com>
> > > ---
> > >  net/ipv4/ipmr.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
> > > index 866ee89..48df3cc 100644
> > > --- a/net/ipv4/ipmr.c
> > > +++ b/net/ipv4/ipmr.c
> > > @@ -936,7 +936,9 @@ static void ipmr_cache_resolve(struct net *net, struct mr_table *mrt,
> > >  
> > >  			rtnl_unicast(skb, net, NETLINK_CB(skb).portid);
> > >  		} else {
> > > +			preempt_disable();
> > >  			ip_mr_forward(net, mrt, skb, c, 0);
> > > +			preempt_enable();
> > >  		}
> > >  	}
> > >  }
> > 
> > I do not believe this fix is correct.
> 
> Yes, sorry.  I should have suggested local_bh_disable instead.
> 
> > Better replace the
> > IP_INC_STATS_BH() by IP_INC_STATS()
> >
> > and IP_ADD_STATS_BH() by IP_ADD_STATS()
> 
> Hmm, whats the rationale for this?
> 
> Note that IP_ADD_STATS_BH in question is unconditional (not in
> error path).  It seems that its virtually always called from softirq
> except in the setsockopt case.

The naming of the functions is bad if you compare them to e.g.
spin_lock_bh.

STATS_BH can only be used from bottom half and the normal ones (without
_BH) can be called from everywhere. It is a common pattern in the
kernel.

Eric's proposal is correct.

Bye,
Hannes
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