Re: [PATCH] igmp: fix ip_mc_sf_allow race [v3]

[Stephen Hemminger <shemminger@vyatta.com>]

On Wed, 6 Jan 2010 08:40:27 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> > -	if (inet->mc_list == NULL)
> > +	rcu_read_lock();
> > +	if (rcu_dereference(inet->mc_list) == NULL) {
> > +		rcu_read_unlock();
> >  		return;
> > +	}
> > +	rcu_read_unlock();  
> 
> I don't understand what rcu_read_lock() is protecting here.  The
> test is still unstable -- just after finding inet->mc_list non-NULL,
> ip_mc_leave_group() might cause it to become NULL.
> 
> Is there a need to protect sock_net(sk)?  (I don't believe so, but then
> again, I don't claim to understand locking in Linux networking.)
> If there is no need, it should be possible to drop the rcu_read_lock(),
> rcu_read_unlock(), and rcu_dereference() above.  (You might want them
> for documentation purposes, as they aren't hurting anything, just
> wondering what the intent is.)

I think code is trying to avoid looking at mc_list if no multicast
addresses. But it is an unsafe check.

If mc_list was just converted to list_head this would all be clearer