mld: convert ip6_sf_list to RCU

The ip6_sf_list has been protected by mca_lock(spin_lock) so that the
critical section is atomic context. In order to switch this context,
changing locking is needed. The ip6_sf_list actually already protected
by RTNL So if it's converted to use RCU, its control path context can
be switched to sleepable.
But It doesn't remove mca_lock yet because ifmcaddr6 isn't converted
to RCU yet. So, It's not fully converted to the sleepable context.

Suggested-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/if_inet6.h

@@ -97,12 +97,13 @@ struct ipv6_mc_socklist {
 };
 
 struct ip6_sf_list {
-	struct ip6_sf_list	*sf_next;
+	struct ip6_sf_list __rcu *sf_next;
 	struct in6_addr		sf_addr;
 	unsigned long		sf_count[2];	/* include/exclude counts */
 	unsigned char		sf_gsresp;	/* include in g & s response? */
 	unsigned char		sf_oldin;	/* change state */
 	unsigned char		sf_crcount;	/* retrans. left to send */
+	struct rcu_head		rcu;
 };
 
 #define MAF_TIMER_RUNNING	0x01
@@ -115,8 +116,8 @@ struct ifmcaddr6 {
 	struct in6_addr		mca_addr;
 	struct inet6_dev	*idev;
 	struct ifmcaddr6	*next;
-	struct ip6_sf_list	*mca_sources;
-	struct ip6_sf_list	*mca_tomb;
+	struct ip6_sf_list	__rcu *mca_sources;
+	struct ip6_sf_list	__rcu *mca_tomb;
 	unsigned int		mca_sfmode;
 	unsigned char		mca_crcount;
 	unsigned long		mca_sfcount[2];