Re: [PATCH net] ipv6: fix suspecious RCU usage warning

[Ido Schimmel <idosch@idosch.org>]

On Mon, Mar 08, 2021 at 07:47:31PM -0700, David Ahern wrote:
> [ cc Ido and Petr ]
> 
> On 3/8/21 12:21 PM, Wei Wang wrote:
> > diff --git a/include/net/nexthop.h b/include/net/nexthop.h
> > index 7bc057aee40b..48956b144689 100644
> > --- a/include/net/nexthop.h
> > +++ b/include/net/nexthop.h
> > @@ -410,31 +410,39 @@ static inline struct fib_nh *fib_info_nh(struct fib_info *fi, int nhsel)
> >  int fib6_check_nexthop(struct nexthop *nh, struct fib6_config *cfg,
> >  		       struct netlink_ext_ack *extack);
> >  
> > -static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
> > +static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh,
> > +					      bool bh_disabled)
> 
> Hi Wei: I would prefer not to have a second argument to nexthop_fib6_nh
> for 1 code path, and a control path at that.
> 
> >  {
> >  	struct nh_info *nhi;
> >  
> >  	if (nh->is_group) {
> >  		struct nh_group *nh_grp;
> >  
> > -		nh_grp = rcu_dereference_rtnl(nh->nh_grp);
> > +		if (bh_disabled)
> > +			nh_grp = rcu_dereference_bh_rtnl(nh->nh_grp);
> > +		else
> > +			nh_grp = rcu_dereference_rtnl(nh->nh_grp);
> >  		nh = nexthop_mpath_select(nh_grp, 0);
> >  		if (!nh)
> >  			return NULL;
> >  	}
> >  
> > -	nhi = rcu_dereference_rtnl(nh->nh_info);
> > +	if (bh_disabled)
> > +		nhi = rcu_dereference_bh_rtnl(nh->nh_info);
> > +	else
> > +		nhi = rcu_dereference_rtnl(nh->nh_info);
> >  	if (nhi->family == AF_INET6)
> >  		return &nhi->fib6_nh;
> >  
> >  	return NULL;
> >  }
> >  
> 
> I am wary of duplicating code, but this helper is simple enough that it
> should be ok with proper documentation.
> 
> Ido/Petr: I think your resilient hashing patch set touches this helper.
> How ugly does it get to have a second version?

It actually doesn't touch this helper. Looks fine to me:

diff --git a/include/net/nexthop.h b/include/net/nexthop.h
index ba94868a21d5..6df9c12546fd 100644
--- a/include/net/nexthop.h
+++ b/include/net/nexthop.h
@@ -496,6 +496,26 @@ static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
 	return NULL;
 }
 
+static inline struct fib6_nh *nexthop_fib6_nh_bh(struct nexthop *nh)
+{
+	struct nh_info *nhi;
+
+	if (nh->is_group) {
+		struct nh_group *nh_grp;
+
+		nh_grp = rcu_dereference_bh(nh->nh_grp);
+		nh = nexthop_mpath_select(nh_grp, 0);
+		if (!nh)
+			return NULL;
+	}
+
+	nhi = rcu_dereference_bh(nh->nh_info);
+	if (nhi->family == AF_INET6)
+		return &nhi->fib6_nh;
+
+	return NULL;
+}
+
 static inline struct net_device *fib6_info_nh_dev(struct fib6_info *f6i)
 {
 	struct fib6_nh *fib6_nh;
diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index ef9d022e693f..679699e953f1 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -2486,7 +2486,7 @@ static int ipv6_route_native_seq_show(struct seq_file *seq, void *v)
 	const struct net_device *dev;
 
 	if (rt->nh)
-		fib6_nh = nexthop_fib6_nh(rt->nh);
+		fib6_nh = nexthop_fib6_nh_bh(rt->nh);
 
 	seq_printf(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);