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 <oleg@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/20231117164846.GA10410@redhat.com/

diff --git a/net/rxrpc/conn_service.c b/net/rxrpc/conn_service.c
index 89ac05a711a427afa94f56cdc318cd7a7add7812..39c908a3ca6e89682b566a704b2dc088943bafbe 100644 (file)
--- a/net/rxrpc/conn_service.c
+++ b/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);