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Message-ID: <20231116131849.GA27763@redhat.com>
Date: Thu, 16 Nov 2023 14:18:49 +0100
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>,
Al Viro <viro@...iv.linux.org.uk>
Cc: Chuck Lever <chuck.lever@...cle.com>,
linux-afs@...ts.infradead.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather
than read_seqbegin_or_lock()
David, Al,
So do you agree that
- the usage of read_seqbegin_or_lock/need_seqretry in
this code makes no sense because read_seqlock_excl()
is not possible
- this patch doesn't change the current behaviour but
simplifies the code and makes it more clear
?
On 10/27, Oleg Nesterov wrote:
>
> 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
>
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