rxrpc_find_service_conn_rcu: fix the usage of read_seqbegin_or_lock()

rxrpc_find_service_conn_rcu() should make the "seq" counter odd on the
second pass, otherwise read_seqbegin_or_lock() never takes the lock.

Signed-off-by: Oleg Nesterov <[email protected]>
Signed-off-by: David Howells <[email protected]>
cc: Marc Dionne <[email protected]>
cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]/

Author: oleg-nesterov

net/rxrpc/conn_service.c

@@ -25,7 +25,7 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer,
 	struct rxrpc_conn_proto k;
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
 	struct rb_node *p;
-	unsigned int seq = 0;
+	unsigned int seq = 1;
 
 	k.epoch	= sp->hdr.epoch;
 	k.cid	= sp->hdr.cid & RXRPC_CIDMASK;
@@ -35,6 +35,7 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer,
 		 * under just the RCU read lock, so we have to check for
 		 * changes.
 		 */
+		seq++; /* 2 on the 1st/lockless path, otherwise odd */
 		read_seqbegin_or_lock(&peer->service_conn_lock, &seq);
 
 		p = rcu_dereference_raw(peer->service_conns.rb_node);