net/l2tp: convert tunnel rwlock_t to rcu

Previously commit e02d494d2c60 ("l2tp: Convert rwlock to RCU") converted
most, but not all, rwlock instances in the l2tp subsystem to RCU.

The remaining rwlock protects the per-tunnel hashlist of sessions which
is used for session lookups in the UDP-encap data path.

Convert the remaining rwlock to rcu to improve performance of UDP-encap
tunnels.

Note that the tunnel and session, which both live on RCU-protected
lists, use slightly different approaches to incrementing their refcounts
in the various getter functions.

The tunnel has to use refcount_inc_not_zero because the tunnel shutdown
process involves dropping the refcount to zero prior to synchronizing
RCU readers (via. kfree_rcu).

By contrast, the session shutdown removes the session from the list(s)
it is on, synchronizes with readers, and then decrements the session
refcount.  Since the getter functions increment the session refcount
with the RCU read lock held we prevent getters seeing a zero session
refcount, and therefore don't need to use refcount_inc_not_zero.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 25 insertions, 27 deletions

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 93271a2632b8..7499c51b1850 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -250,15 +250,15 @@ struct l2tp_session *l2tp_tunnel_get_session(struct l2tp_tunnel *tunnel,
 
 	session_list = l2tp_session_id_hash(tunnel, session_id);
 
-	read_lock_bh(&tunnel->hlist_lock);
-	hlist_for_each_entry(session, session_list, hlist)
+	rcu_read_lock_bh();
+	hlist_for_each_entry_rcu(session, session_list, hlist)
 		if (session->session_id == session_id) {
 			l2tp_session_inc_refcount(session);
-			read_unlock_bh(&tunnel->hlist_lock);
+			rcu_read_unlock_bh();
 
 			return session;
 		}
-	read_unlock_bh(&tunnel->hlist_lock);
+	rcu_read_unlock_bh();
 
 	return NULL;
 }
@@ -291,18 +291,18 @@ struct l2tp_session *l2tp_session_get_nth(struct l2tp_tunnel *tunnel, int nth)
 	struct l2tp_session *session;
 	int count = 0;
 
-	read_lock_bh(&tunnel->hlist_lock);
+	rcu_read_lock_bh();
 	for (hash = 0; hash < L2TP_HASH_SIZE; hash++) {
-		hlist_for_each_entry(session, &tunnel->session_hlist[hash], hlist) {
+		hlist_for_each_entry_rcu(session, &tunnel->session_hlist[hash], hlist) {
 			if (++count > nth) {
 				l2tp_session_inc_refcount(session);
-				read_unlock_bh(&tunnel->hlist_lock);
+				rcu_read_unlock_bh();
 				return session;
 			}
 		}
 	}
 
-	read_unlock_bh(&tunnel->hlist_lock);
+	rcu_read_unlock_bh();
 
 	return NULL;
 }
@@ -347,7 +347,7 @@ int l2tp_session_register(struct l2tp_session *session,
 
 	head = l2tp_session_id_hash(tunnel, session->session_id);
 
-	write_lock_bh(&tunnel->hlist_lock);
+	spin_lock_bh(&tunnel->hlist_lock);
 	if (!tunnel->acpt_newsess) {
 		err = -ENODEV;
 		goto err_tlock;
@@ -384,8 +384,8 @@ int l2tp_session_register(struct l2tp_session *session,
 		l2tp_tunnel_inc_refcount(tunnel);
 	}
 
-	hlist_add_head(&session->hlist, head);
-	write_unlock_bh(&tunnel->hlist_lock);
+	hlist_add_head_rcu(&session->hlist, head);
+	spin_unlock_bh(&tunnel->hlist_lock);
 
 	trace_register_session(session);
 
@@ -394,7 +394,7 @@ int l2tp_session_register(struct l2tp_session *session,
 err_tlock_pnlock:
 	spin_unlock_bh(&pn->l2tp_session_hlist_lock);
 err_tlock:
-	write_unlock_bh(&tunnel->hlist_lock);
+	spin_unlock_bh(&tunnel->hlist_lock);
 
 	return err;
 }
@@ -1170,9 +1170,9 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 	/* Remove the session from core hashes */
 	if (tunnel) {
 		/* Remove from the per-tunnel hash */
-		write_lock_bh(&tunnel->hlist_lock);
-		hlist_del_init(&session->hlist);
-		write_unlock_bh(&tunnel->hlist_lock);
+		spin_lock_bh(&tunnel->hlist_lock);
+		hlist_del_init_rcu(&session->hlist);
+		spin_unlock_bh(&tunnel->hlist_lock);
 
 		/* For L2TPv3 we have a per-net hash: remove from there, too */
 		if (tunnel->version != L2TP_HDR_VER_2) {
@@ -1181,8 +1181,9 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 			spin_lock_bh(&pn->l2tp_session_hlist_lock);
 			hlist_del_init_rcu(&session->global_hlist);
 			spin_unlock_bh(&pn->l2tp_session_hlist_lock);
-			synchronize_rcu();
 		}
+
+		synchronize_rcu();
 	}
 }
 
@@ -1190,22 +1191,19 @@ static void l2tp_session_unhash(struct l2tp_session *session)
  */
 static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
 {
-	int hash;
-	struct hlist_node *walk;
-	struct hlist_node *tmp;
 	struct l2tp_session *session;
+	int hash;
 
-	write_lock_bh(&tunnel->hlist_lock);
+	spin_lock_bh(&tunnel->hlist_lock);
 	tunnel->acpt_newsess = false;
 	for (hash = 0; hash < L2TP_HASH_SIZE; hash++) {
 again:
-		hlist_for_each_safe(walk, tmp, &tunnel->session_hlist[hash]) {
-			session = hlist_entry(walk, struct l2tp_session, hlist);
-			hlist_del_init(&session->hlist);
+		hlist_for_each_entry_rcu(session, &tunnel->session_hlist[hash], hlist) {
+			hlist_del_init_rcu(&session->hlist);
 
-			write_unlock_bh(&tunnel->hlist_lock);
+			spin_unlock_bh(&tunnel->hlist_lock);
 			l2tp_session_delete(session);
-			write_lock_bh(&tunnel->hlist_lock);
+			spin_lock_bh(&tunnel->hlist_lock);
 
 			/* Now restart from the beginning of this hash
 			 * chain.  We always remove a session from the
@@ -1215,7 +1213,7 @@ again:
 			goto again;
 		}
 	}
-	write_unlock_bh(&tunnel->hlist_lock);
+	spin_unlock_bh(&tunnel->hlist_lock);
 }
 
 /* Tunnel socket destroy hook for UDP encapsulation */
@@ -1408,7 +1406,7 @@ int l2tp_tunnel_create(int fd, int version, u32 tunnel_id, u32 peer_tunnel_id,
 
 	tunnel->magic = L2TP_TUNNEL_MAGIC;
 	sprintf(&tunnel->name[0], "tunl %u", tunnel_id);
-	rwlock_init(&tunnel->hlist_lock);
+	spin_lock_init(&tunnel->hlist_lock);
 	tunnel->acpt_newsess = true;
 
 	tunnel->encap = encap;