Re: [Patch net] ipv6,mcast: always hold idev->lock before mca_lock

[Cong Wang <amwang@redhat.com>]

On Sat, 2013-05-11 at 16:11 -0700, David Miller wrote:
> From: Cong Wang <amwang@redhat.com>
> Date: Wed,  8 May 2013 15:41:54 +0800
> 
> > @@ -1369,7 +1370,7 @@ static struct sk_buff *mld_newpack(struct net_device *dev, int size)
> >  
> >  	skb_reserve(skb, hlen);
> >  
> > -	if (ipv6_get_lladdr(dev, &addr_buf, IFA_F_TENTATIVE)) {
> > +	if (__ipv6_get_lladdr(idev, &addr_buf, IFA_F_TENTATIVE)) {
> >  		/* <draft-ietf-magma-mld-source-05.txt>:
> >  		 * use unspecified address as the source address
> >  		 * when a valid link-local address is not available.
> 
> You aren't necessarily going to be holding idev->lock, therefore you can't
> just do a lockless traversal of idev->addr_list here.
> 
> Yes, you can elide the rcu_read_lock() because you have a known reference
> to 'idev' in these paths, but you can't get rid of the address list locking
> altogether.

(Apologize for the delay...)

The upper callers of mld_newpack() already take
read_lock_bh(&idev->lock): 


mld_newpack() ...-> add_grec() -|-> mld_send_cr()     LOCKED
                                |-> mld_send_report() LOCKED


therefore it is safe to travel idev->addr_list locklessly here.