igmp: RCU conversion of in_dev->mc_list

in_dev->mc_list is protected by one rwlock (in_dev->mc_list_lock).

This can easily be converted to a RCU protection.

Writers hold RTNL, so mc_list_lock is removed, not replaced by a
spinlock.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Cypher Wu <cypher.w@gmail.com>
Cc: Américo Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/igmp.h

@@ -167,10 +167,10 @@ struct ip_sf_socklist {
  */
 
 struct ip_mc_socklist {
-	struct ip_mc_socklist	*next;
+	struct ip_mc_socklist __rcu *next_rcu;
 	struct ip_mreqn		multi;
 	unsigned int		sfmode;		/* MCAST_{INCLUDE,EXCLUDE} */
-	struct ip_sf_socklist	*sflist;
+	struct ip_sf_socklist __rcu	*sflist;
 	struct rcu_head		rcu;
 };
 
@@ -186,11 +186,14 @@ struct ip_sf_list {
 struct ip_mc_list {
 	struct in_device	*interface;
 	__be32			multiaddr;
+	unsigned int		sfmode;
 	struct ip_sf_list	*sources;
 	struct ip_sf_list	*tomb;
-	unsigned int		sfmode;
 	unsigned long		sfcount[2];
+	union {
 		struct ip_mc_list *next;
+		struct ip_mc_list __rcu *next_rcu;
+	};
 	struct timer_list	timer;
 	int			users;
 	atomic_t		refcnt;
@@ -201,6 +204,7 @@ struct ip_mc_list {
 	char			loaded;
 	unsigned char		gsquery;	/* check source marks? */
 	unsigned char		crcount;
+	struct rcu_head		rcu;
 };
 
 /* V3 exponential field decoding */

include/linux/inetdevice.h

@@ -52,8 +52,7 @@ struct in_device {
 	atomic_t		refcnt;
 	int			dead;
 	struct in_ifaddr	*ifa_list;	/* IP ifaddr chain		*/
-	rwlock_t		mc_list_lock;
-	struct ip_mc_list	*mc_list;	/* IP multicast filter chain    */
+	struct ip_mc_list __rcu	*mc_list;	/* IP multicast filter chain    */
 	int			mc_count;	/* Number of installed mcasts	*/
 	spinlock_t		mc_tomb_lock;
 	struct ip_mc_list	*mc_tomb;

include/net/inet_sock.h

@@ -141,7 +141,7 @@ struct inet_sock {
 				nodefrag:1;
 	int			mc_index;
 	__be32			mc_addr;
-	struct ip_mc_socklist	*mc_list;
+	struct ip_mc_socklist __rcu	*mc_list;
 	struct {
 		unsigned int		flags;
 		unsigned int		fragsize;