Re: [PATCH] IB/core: Suppress a sparse warning

["Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

On Mon, Mar 10, 2014 at 04:02:13PM +0100, Yann Droneaud wrote:
> Hi,
> 
> Le lundi 10 mars 2014 à 15:26 +0100, Bart Van Assche a écrit :
> > On 03/10/14 14:33, Yann Droneaud wrote:
> > > Le lundi 10 mars 2014 à 13:22 +0100, Bart Van Assche a écrit :
> > >> Suppress the following sparse warning:
> > >> include/rdma/ib_addr.h:187:24: warning: cast removes address space of expression
> > > 
> > > You should explain why there's a warning here, and why is it safe to
> > > cast. (I believe it's related to RCU domain ?)
> > 
> > Hello Yann,
> > 
> > Now that I've had a closer look at the code in include/rdma/ib_addr.h,
> > that code probably isn't safe. How about the (untested) patch below ?
> > 
> 
> Thanks for investigating.
> 
> I'm not an expert in RCU, but I believe it then miss the RCU annotations
> around the RCU reader section (ensure correct ordering if I recall
> correctly).
> 
> Cc: "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>

If the rcu_read_lock() isn't supplied by all callers to this function,
then yes, it needs to be supplied as Yann shows below.

The CONFIG_PROVE_RCU=y Kconfig option can help determine that they are
needed, but of course cannot prove that they are not needed, at least
not unless you have a workload that exercises all possible calls to
this function.

							Thanx, Paul

> > diff --git a/include/rdma/ib_addr.h b/include/rdma/ib_addr.h
> > index ce55906..5a416ac 100644
> > --- a/include/rdma/ib_addr.h
> > +++ b/include/rdma/ib_addr.h
> > @@ -184,7 +184,7 @@ static inline void iboe_addr_get_sgid(struct rdma_dev_addr *dev_addr,
> >  
> >  	dev = dev_get_by_index(&init_net, dev_addr->bound_dev_if);
> >  	if (dev) {
> 
> +		rcu_read_lock();
> 
> > -		ip4 = (struct in_device *)dev->ip_ptr;
> > +		ip4 = __in_dev_get_rcu(dev);
> 
> >  		if (ip4 && ip4->ifa_list && ip4->ifa_list->ifa_address)
> >  			ipv6_addr_set_v4mapped(ip4->ifa_list->ifa_address,
> >  					       (struct in6_addr *)gid);
> 
> +		rcu_read_unlock();
> 
> 
> Regards.
> 
> -- 
> Yann Droneaud
> OPTEYA
> 
> 

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