linux-kernel - [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather than read_seqbegin_or

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Message-ID: <20231027095842.GA30868@redhat.com>
Date:   Fri, 27 Oct 2023 11:58:42 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     David Howells <dhowells@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     Chuck Lever <chuck.lever@...cle.com>,
        linux-afs@...ts.infradead.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather than
 read_seqbegin_or_lock()

read_seqbegin_or_lock() makes no sense unless you make "seq" odd
after the lockless access failed. See thread_group_cputime() as
an example, note that it does nextseq = 1 for the 2nd round.

So this code can use read_seqbegin() without changing the current
behaviour.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 net/rxrpc/conn_service.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/rxrpc/conn_service.c b/net/rxrpc/conn_service.c
index 89ac05a711a4..bfafe58681d9 100644
--- 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;
 
 	k.epoch	= sp->hdr.epoch;
 	k.cid	= sp->hdr.cid & RXRPC_CIDMASK;
@@ -35,7 +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.
 		 */
-		read_seqbegin_or_lock(&peer->service_conn_lock, &seq);
+		seq = read_seqbegin(&peer->service_conn_lock);
 
 		p = rcu_dereference_raw(peer->service_conns.rb_node);
 		while (p) {
@@ -49,9 +49,8 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer,
 				break;
 			conn = NULL;
 		}
-	} while (need_seqretry(&peer->service_conn_lock, seq));
+	} while (read_seqretry(&peer->service_conn_lock, seq));
 
-	done_seqretry(&peer->service_conn_lock, seq);
 	_leave(" = %d", conn ? conn->debug_id : -1);
 	return conn;
 }
-- 
2.25.1.362.g51ebf55