netdev - Re: suspicious RCU usage in net/ipv4/ip

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Date:	Sun, 12 Jan 2014 09:44:57 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <cwang@...pensource.com>
Cc:	Tom Herbert <therbert@...gle.com>, netdev <netdev@...r.kernel.org>
Subject: Re: suspicious RCU usage in net/ipv4/ip_tunnel.c:80

On Sat, 2014-01-11 at 11:15 -0800, Cong Wang wrote:
> On Fri, Jan 10, 2014 at 5:21 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> >
> > Nope, the synchronize_rcu() is not needed here.
> 
> OK.
> 
> >
> > Please use following sparse ready patch, thanks :
> >
> > diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> > index d3929a69f008..6eda759b5c4b 100644
> > --- a/net/ipv4/ip_tunnel.c
> > +++ b/net/ipv4/ip_tunnel.c
> > @@ -77,10 +77,11 @@ static inline void __tunnel_dst_set(struct ip_tunnel_dst *idst,
> >                 dst = NULL;
> >
> >         spin_lock_bh(&idst->lock);
> > -       old_dst = rcu_dereference(idst->dst);
> > +       old_dst = rcu_dereference_protected(idst->dst,
> > +                                           lockdep_is_held(&idst->lock));
> >         rcu_assign_pointer(idst->dst, dst);
> > -       dst_release(old_dst);
> >         spin_unlock_bh(&idst->lock);
> > +       dst_release(old_dst);
> 
> Do you really need a rcu_dereference*() here? We don't dereference
> it inside spin_lock protection.

Please read rcu_dereference_protected() documentation in
include/linux/rcupdate.h

Also you can run sparse, with CONFIG_SPARSE_RCU_POINTER=y in
your .config

make C=2 net/ipv4/ip_tunnel.o

And then you'll know the answer to this question.




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