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

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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
>