Subject: Re: [PATCH] sctp: Add RCU protection to assoc->transport_addr_list

From: Vlad Yasevich <[email-masked]>
Date: Thu, 06 Dec 2012 13:35:49 -0500

On 12/06/2012 01:15 PM, Thomas Graf wrote:
> peer.transport_addr_list is currently only protected by sk_sock
> which is inpractical to acquire for procfs dumping purposes.
>
> This patch adds RCU protection allowing for the procfs readers to
> enter RCU read-side critical sections.
>
> Modification of the list continues to be serialized via sk_lock.
>
> Cc: Vlad Yasevich <[email-masked]>
> Cc: Neil Horman <[email-masked]>
> Signed-off-by: Thomas Graf <[email-masked]>
> ---
>   include/net/sctp/structs.h |  2 ++
>   net/sctp/associola.c       |  4 ++--
>   net/sctp/proc.c            |  8 ++++++--
>   net/sctp/transport.c       | 18 +++++++++++++-----
>   4 files changed, 23 insertions(+), 9 deletions(-)
>
> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
> index 64158aa..5d6987b 100644
> --- a/include/net/sctp/structs.h
> +++ b/include/net/sctp/structs.h
> @@ -948,6 +948,8 @@ struct sctp_transport {
>
>   	/* 64-bit random number sent with heartbeat. */
>   	__u64 hb_nonce;
> +
> +	struct rcu_head rcu;
>   };
>
>   struct sctp_transport *sctp_transport_new(struct net *, const union sctp_addr *,
> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> index b1ef3bc..c826bb8 100644
> --- a/net/sctp/associola.c
> +++ b/net/sctp/associola.c
> @@ -565,7 +565,7 @@ void sctp_assoc_rm_peer(struct sctp_association *asoc,
>   		sctp_assoc_update_retran_path(asoc);
>
>   	/* Remove this peer from the list. */
> -	list_del(&peer->transports);
> +	list_del_rcu(&peer->transports);
>
>   	/* Get the first transport of asoc. */
>   	pos = asoc->peer.transport_addr_list.next;
> @@ -765,7 +765,7 @@ struct sctp_transport *sctp_assoc_add_peer(struct sctp_association *asoc,
>   	peer->state = peer_state;
>
>   	/* Attach the remote transport to our asoc.  */
> -	list_add_tail(&peer->transports, &asoc->peer.transport_addr_list);
> +	list_add_tail_rcu(&peer->transports, &asoc->peer.transport_addr_list);
>   	asoc->peer.transport_count++;
>
>   	/* If we do not yet have a primary path, set one.  */
> diff --git a/net/sctp/proc.c b/net/sctp/proc.c
> index ec9b0c8..36a3f9d 100644
> --- a/net/sctp/proc.c
> +++ b/net/sctp/proc.c
> @@ -159,7 +159,8 @@ static void sctp_seq_dump_remote_addrs(struct seq_file *seq, struct sctp_associa
>   	struct sctp_af *af;
>
>   	primary = &assoc->peer.primary_addr;
> -	list_for_each_entry(transport, &assoc->peer.transport_addr_list,
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(transport, &assoc->peer.transport_addr_list,
>   			transports) {
>   		addr = &transport->ipaddr;
>   		af = sctp_get_af_specific(addr->sa.sa_family);
> @@ -168,6 +169,7 @@ static void sctp_seq_dump_remote_addrs(struct seq_file *seq, struct sctp_associa
>   		}
>   		af->seq_dump_addr(seq, addr);
>   	}
> +	rcu_read_unlock();
>   }
>
>   static void * sctp_eps_seq_start(struct seq_file *seq, loff_t *pos)
> @@ -438,11 +440,12 @@ static int sctp_remaddr_seq_show(struct seq_file *seq, void *v)
>   	head = &sctp_assoc_hashtable[hash];
>   	sctp_local_bh_disable();
>   	read_lock(&head->lock);
> +	rcu_read_lock();
>   	sctp_for_each_hentry(epb, node, &head->chain) {
>   		if (!net_eq(sock_net(epb->sk), seq_file_net(seq)))
>   			continue;
>   		assoc = sctp_assoc(epb);
> -		list_for_each_entry(tsp, &assoc->peer.transport_addr_list,
> +		list_for_each_entry_rcu(tsp, &assoc->peer.transport_addr_list,
>   					transports) {
>   			/*
>   			 * The remote address (ADDR)
> @@ -489,6 +492,7 @@ static int sctp_remaddr_seq_show(struct seq_file *seq, void *v)
>   		}
>   	}
>
> +	rcu_read_unlock();
>   	read_unlock(&head->lock);
>   	sctp_local_bh_enable();
>
> diff --git a/net/sctp/transport.c b/net/sctp/transport.c
> index 206cf52..1295aec 100644
> --- a/net/sctp/transport.c
> +++ b/net/sctp/transport.c
> @@ -163,13 +163,11 @@ void sctp_transport_free(struct sctp_transport *transport)
>   	sctp_transport_put(transport);
>   }
>
> -/* Destroy the transport data structure.
> - * Assumes there are no more users of this structure.
> - */
> -static void sctp_transport_destroy(struct sctp_transport *transport)
> +static void sctp_transport_destroy_rcu(struct rcu_head *head)
>   {
> -	SCTP_ASSERT(transport->dead, "Transport is not dead", return);
> +	struct sctp_transport *transport;
>
> +	transport = container_of(head, struct sctp_transport, rcu);
>   	if (transport->asoc)
>   		sctp_association_put(transport->asoc);
>
> @@ -180,6 +178,16 @@ static void sctp_transport_destroy(struct sctp_transport *transport)
>   	SCTP_DBG_OBJCNT_DEC(transport);
>   }
>
> +/* Destroy the transport data structure.
> + * Assumes there are no more users of this structure.
> + */
> +static void sctp_transport_destroy(struct sctp_transport *transport)
> +{
> +	SCTP_ASSERT(transport->dead, "Transport is not dead", return);
> +
> +	call_rcu(&transport->rcu, sctp_transport_destroy_rcu);
> +}
> +
>   /* Start T3_rtx timer if it is not already running and update the heartbeat
>    * timer.  This routine is called every time a DATA chunk is sent.
>    */
>

We may want to mark transports as dead sooner.  Probably right about the 
time we pull them off the list.  When displaying, we may want to
look at transport->dead, and skip them.  It will reduce the probability
that we would be looking at a transport that's about to go away.

Otherwise, looks very nice.

-vlad