Subject: Re: [IPV4 3/5] fib_trie: dump doesnt use RCU

From: Patrick McHardy <[email-masked]>
Date: Thu, 24 Jan 2008 07:41:08 +0100

David Miller wrote:
> From: Stephen Hemminger <[email-masked]>
> Date: Wed, 23 Jan 2008 14:48:47 -0800
>
>   
>> Since fib dump (via netlink) holds the RTNL mutex, it is unnecessary
>> to use RCU, and it is impossible to get truncated (-EBUSY) result.
>>
>> Signed-off-by: Stephen Hemminger <[email-masked]>
>>     
>
> You tested this patch, right? :-/
>
> The whole reason we need the nlk->cb[] state is to hold things across
> multiple recvmsg() calls that might be necessary to obtain the full
> dump.
>
> rtnetlink goes:
>
> 	rtnl_lock();
> 	netlink_rcv_skb(skb, &rtnetlink_rcv_msg);
>  ...
> static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
> {
>  ...
> 	if (kind == 2 && nlh->nlmsg_flags&NLM_F_DUMP) {
> 		struct sock *rtnl;
> 		rtnl_dumpit_func dumpit;
>
> 		dumpit = rtnl_get_dumpit(family, type);
> 		if (dumpit == NULL)
> 			return -EOPNOTSUPP;
>
> 		__rtnl_unlock();
> 		rtnl = net->rtnl;
> 		err = netlink_dump_start(rtnl, skb, nlh, dumpit, NULL);
> 		rtnl_lock();
> 		return err;
>
> (NOTE: Drops RTNL semaphore for netlink_dump_start() call)
>
>  ...
> int netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
> 		       struct nlmsghdr *nlh,
> 		       int (*dump)(struct sk_buff *skb,
> 				   struct netlink_callback *),
> 		       int (*done)(struct netlink_callback *))
> {
>  ...
> 	cb->dump = dump;
> 	cb->done = done;
> 	cb->nlh = nlh;
> 	atomic_inc(&skb->users);
> 	cb->skb = skb;
>  ...
> 	mutex_lock(nlk->cb_mutex);
>  ...
> 	nlk->cb = cb;
> 	mutex_unlock(nlk->cb_mutex);
>
> 	netlink_dump(sk);
>  ...
>
> static int netlink_dump(struct sock *sk)
> {
>  ...
> 	mutex_lock(nlk->cb_mutex);
>  ...
> 	len = cb->dump(skb, cb);
>
> 	if (len > 0) {
> 		mutex_unlock(nlk->cb_mutex);
> 		skb_queue_tail(&sk->sk_receive_queue, skb);
> 		sk->sk_data_ready(sk, len);
> 		return 0;
> 	}
>
> (NOTE: Therefore cb->dump() runs without RTNL semaphore held)
>
>  ...
> static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
> 			   struct msghdr *msg, size_t len,
> 			   int flags)
> {
>  ...
> 	if (nlk->cb && atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2)
> 		netlink_dump(sk);
>  ...
>
> Therefore, that RTNL assertion you added should have triggered on any
> dump you may have tried since ->dump() is always invoked without the
> RTNL semaphore since rtnetlink drops it around the ->dump() call and
> the call chain for this fib_trie cause would be:
>
> inet_dump_fib()
>  fn_trie_dump()
>
> and nothing in that code path retakes the RTNL semaphore.

Actually we're always holding the rtnl during dumps, nlk->cb_mutex points
to rtnl_mutex in case of rtnetlink. It used to be held only during the first
->dump invocation and not on continuations, but I changed this a few 
versions
ago.