netdev - Re: [PATCH] sctp: don't break the loop while meeting the active

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Date:	Tue, 12 Mar 2013 07:30:50 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Xufeng Zhang <xufengzhang.main@...il.com>
Cc:	Xufeng Zhang <xufeng.zhang@...driver.com>, vyasevich@...il.com,
	davem@...emloft.net, linux-sctp@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sctp: don't break the loop while meeting the active_path
 so as to find the matched transport

On Tue, Mar 12, 2013 at 10:24:02AM +0800, Xufeng Zhang wrote:
> >>
> >> Thanks for your review, Neil!
> >>
> >> I know what you mean, yes, it's most probably that the searched TSN was
> >> transmitted in the currently active_path, but what should we do if not.
> >>
> >> Check the comment in sctp_assoc_lookup_tsn() function:
> >> /* Let's be hopeful and check the active_path first. */
> >> /* If not found, go search all the other transports. */
> >>
> >> It has checked the active_path first and then traverse all the other
> >> transports in
> >> the transport_addr_list except the active_path.
> >>
> >> So what I want to fix here is the inconsistency between the function
> >> should do and
> >> the code actually does.
> >>
> > I understand what you're doing, and I agree that the fix is functional
> > (Hence
> > my "This works" statement in my last note).  What I'm suggesting is that,
> > since
> > you're messing about in that code anyway that you clean it up while your at
> > it,
> > so that we don't need to have the if (transport == active) check at all.
> > We
> > trade in some extra work in a non-critical path (sctp_assoc_set_primary),
> > for
> > the removal of an extra for loop operation and a conditional check in a
> > much
> > hotter path.  Something like this (completely untested), is what I was
> > thinking
> 
> Aha, seems I have some misunderstanding previously, now I got your point.
> Yeah, it's better to do the clean up by this way, and this fix looks fine to me,
> but I didn't have a test case to test this, actually this problem was detected
> by code review, so I would like to leave the rest of this work to
> determine by you.
> 
> Thank you very much for your clarification!
> 
Ok, I'll try set up a test for this today
Neil

> 
> Thanks,
> Xufeng
> 
> >
> >
> > diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> > index 43cd0dd..8ae873c 100644
> > --- a/net/sctp/associola.c
> > +++ b/net/sctp/associola.c
> > @@ -505,6 +505,9 @@ void sctp_assoc_set_primary(struct sctp_association
> > *asoc,
> >
> >  	asoc->peer.primary_path = transport;
> >
> > +	list_del_rcu(&transport->transports);
> > +	list_add_rcu(&transport->transports, &asoc->peer.transport_addr_list);
> > +
> >  	/* Set a default msg_name for events. */
> >  	memcpy(&asoc->peer.primary_addr, &transport->ipaddr,
> >  	       sizeof(union sctp_addr));
> > @@ -1040,7 +1043,6 @@ struct sctp_chunk *sctp_get_ecne_prepend(struct
> > sctp_association *asoc)
> >  struct sctp_transport *sctp_assoc_lookup_tsn(struct sctp_association
> > *asoc,
> >  					     __u32 tsn)
> >  {
> > -	struct sctp_transport *active;
> >  	struct sctp_transport *match;
> >  	struct sctp_transport *transport;
> >  	struct sctp_chunk *chunk;
> > @@ -1057,29 +1059,16 @@ struct sctp_transport *sctp_assoc_lookup_tsn(struct
> > sctp_association *asoc,
> >  	 * The general strategy is to search each transport's transmitted
> >  	 * list.   Return which transport this TSN lives on.
> >  	 *
> > -	 * Let's be hopeful and check the active_path first.
> > -	 * Another optimization would be to know if there is only one
> > -	 * outbound path and not have to look for the TSN at all.
> > +	 * Note, that sctp_assoc_set_primary does a move to front operation
> > +	 * on the active_path transport, so this code implicitly checks
> > +	 * the active_path first, as we most commonly expect to find our TSN
> > +	 * there.
> >  	 *
> >  	 */
> >
> > -	active = asoc->peer.active_path;
> > -
> > -	list_for_each_entry(chunk, &active->transmitted,
> > -			transmitted_list) {
> > -
> > -		if (key == chunk->subh.data_hdr->tsn) {
> > -			match = active;
> > -			goto out;
> > -		}
> > -	}
> > -
> > -	/* If not found, go search all the other transports. */
> >  	list_for_each_entry(transport, &asoc->peer.transport_addr_list,
> >  			transports) {
> >
> > -		if (transport == active)
> > -			break;
> >  		list_for_each_entry(chunk, &transport->transmitted,
> >  				transmitted_list) {
> >  			if (key == chunk->subh.data_hdr->tsn) {
> >
> 
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