l2tp: free sessions using rcu

BugLink: https://bugs.launchpad.net/bugs/2097301

[ Upstream commit d17e89999574aca143dd4ede43e4382d32d98724 ]

l2tp sessions may be accessed under an rcu read lock. Have them freed
via rcu and remove the now unneeded synchronize_rcu when a session is
removed.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>

net/l2tp/l2tp_core.c

@@ -165,7 +165,7 @@ static void l2tp_session_free(struct l2tp_session *session)
 	trace_free_session(session);
 	if (session->tunnel)
 		l2tp_tunnel_dec_refcount(session->tunnel);
-	kfree(session);
+	kfree_rcu(session, rcu);
 }
 
 struct l2tp_tunnel *l2tp_sk_to_tunnel(struct sock *sk)
@@ -1214,8 +1214,6 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 			hlist_del_init_rcu(&session->global_hlist);
 			spin_unlock_bh(&pn->l2tp_session_hlist_lock);
 		}
-
-		synchronize_rcu();
 	}
 }
 

net/l2tp/l2tp_core.h

@@ -70,6 +70,7 @@ struct l2tp_session_cfg {
 struct l2tp_session {
 	int			magic;		/* should be L2TP_SESSION_MAGIC */
 	long			dead;
+	struct rcu_head		rcu;
 
 	struct l2tp_tunnel	*tunnel;	/* back pointer to tunnel context */
 	u32			session_id;