l2tp: delete sessions using work queue

When a tunnel is closed, l2tp_tunnel_closeall closes all sessions in
the tunnel. Move the work of deleting each session to the work queue
so that sessions are deleted using the same codepath whether they are
closed by user API request or their parent tunnel is closing. This
also avoids the locking dance in l2tp_tunnel_closeall where the
tunnel's session list lock was unlocked and relocked in the loop.

In l2tp_exit_net, use drain_workqueue instead of flush_workqueue
because the processing of tunnel_delete work may queue session_delete
work items which must also be processed.
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>

net/l2tp/l2tp_core.c

@@ -1282,18 +1282,8 @@ static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
 
 	spin_lock_bh(&tunnel->list_lock);
 	tunnel->acpt_newsess = false;
-	for (;;) {
-		session = list_first_entry_or_null(&tunnel->session_list,
-						   struct l2tp_session, list);
-		if (!session)
-			break;
-		l2tp_session_inc_refcount(session);
-		list_del_init(&session->list);
-		spin_unlock_bh(&tunnel->list_lock);
+	list_for_each_entry(session, &tunnel->session_list, list)
 		l2tp_session_delete(session);
-		spin_lock_bh(&tunnel->list_lock);
-		l2tp_session_dec_refcount(session);
-	}
 	spin_unlock_bh(&tunnel->list_lock);
 }
 
@@ -1631,18 +1621,31 @@ EXPORT_SYMBOL_GPL(l2tp_tunnel_delete);
 
 void l2tp_session_delete(struct l2tp_session *session)
 {
-	if (test_and_set_bit(0, &session->dead))
-		return;
+	if (!test_and_set_bit(0, &session->dead)) {
+		trace_delete_session(session);
+		l2tp_session_inc_refcount(session);
+		queue_work(l2tp_wq, &session->del_work);
+	}
+}
+EXPORT_SYMBOL_GPL(l2tp_session_delete);
+
+/* Workqueue session deletion function */
+static void l2tp_session_del_work(struct work_struct *work)
+{
+	struct l2tp_session *session = container_of(work, struct l2tp_session,
+						    del_work);
 
-	trace_delete_session(session);
 	l2tp_session_unhash(session);
 	l2tp_session_queue_purge(session);
 	if (session->session_close)
 		(*session->session_close)(session);
 
+	/* drop initial ref */
+	l2tp_session_dec_refcount(session);
+
+	/* drop workqueue ref */
 	l2tp_session_dec_refcount(session);
 }
-EXPORT_SYMBOL_GPL(l2tp_session_delete);
 
 /* We come here whenever a session's send_seq, cookie_len or
  * l2specific_type parameters are set.
@@ -1694,6 +1697,7 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 		INIT_HLIST_NODE(&session->hlist);
 		INIT_LIST_HEAD(&session->clist);
 		INIT_LIST_HEAD(&session->list);
+		INIT_WORK(&session->del_work, l2tp_session_del_work);
 
 		if (cfg) {
 			session->pwtype = cfg->pw_type;
@@ -1751,7 +1755,7 @@ static __net_exit void l2tp_exit_net(struct net *net)
 	rcu_read_unlock_bh();
 
 	if (l2tp_wq)
-		flush_workqueue(l2tp_wq);
+		drain_workqueue(l2tp_wq);
 	rcu_barrier();
 
 	idr_destroy(&pn->l2tp_v2_session_idr);

net/l2tp/l2tp_core.h

@@ -102,6 +102,7 @@ struct l2tp_session {
 	int			reorder_skip;	/* set if skip to next nr */
 	enum l2tp_pwtype	pwtype;
 	struct l2tp_stats	stats;
+	struct work_struct	del_work;
 
 	/* Session receive handler for data packets.
 	 * Each pseudowire implementation should implement this callback in order to