Date:	Wed, 20 May 2015 16:59:24 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	Simon Horman <simon.horman@...ronome.com>
Cc:	Jiri Pirko <jiri@...nulli.us>,
	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 net-next 3/4] rocker: do not make neighbour entry
 changes when preparing transactions

On Wed, May 20, 2015 at 3:32 PM, Simon Horman
<simon.horman@...ronome.com> wrote:
>
> On Wed, May 20, 2015 at 10:37:52AM -0700, Scott Feldman wrote:
> > On Wed, May 20, 2015 at 5:57 AM, Simon Horman
> > <simon.horman@...ronome.com> wrote:
> > > On Wed, May 20, 2015 at 01:17:36PM +0200, Jiri Pirko wrote:
> > >> Wed, May 20, 2015 at 10:46:26AM CEST, simon.horman@...ronome.com wrote:
> > >> >On Wed, May 20, 2015 at 05:36:06PM +0900, Toshiaki Makita wrote:
> > >> >> On 2015/05/20 16:48, Simon Horman wrote:
> > >> >> > On Wed, May 20, 2015 at 03:15:23PM +0900, Toshiaki Makita wrote:
> > >> >> >> On 2015/05/20 14:48, Simon Horman wrote:
> > >> >> ...
> > >> >> >>>  static void _rocker_neigh_add(struct rocker *rocker,
> > >> >> >>> +                            enum switchdev_trans trans,
> > >> >> >>>                              struct rocker_neigh_tbl_entry *entry)
> > >> >> >>>  {
> > >> >> >>> +      if (trans == SWITCHDEV_TRANS_PREPARE)
> > >> >> >>> +              return;
> > >> >> >>>        entry->index = rocker->neigh_tbl_next_index++;
> > >> >> >>
> > >> >> >> Isn't index needed here? It looks to be used in later function call and
> > >> >> >> logging.
> > >> >> >
> > >> >> > Thanks, that does not follow the usual model of setting values
> > >> >> > during the PREPARE (and all other) transaction phase(s).
> > >> >> >
> > >> >> >> How about setting index like this?
> > >> >> >>
> > >> >> >>         entry->index = rocker->neigh_tbl_next_index;
> > >> >> >>         if (trans == PREPARE)
> > >> >> >>                 return;
> > >> >> >>         rocker->neigh_tbl_next_index++;
> > >> >> >>         ...
> > >> >> >
> > >> >> > I am concerned that _rocker_neigh_add() may be called by some other
> > >> >> > caller while a transaction is in process and thus entry->index will
> > >> >> > be inconsistent across callers.
> > >> >> >
> > >> >> > Perhaps we can convince ourselves that all the bases are covered.
> > >> >> > So far my testing has drawn a blank. But the logic seems difficult to
> > >> >> > reason about.
> > >> >> >
> > >> >> > As we are basically allocating an index I suppose what is really needed for
> > >> >> > a correct implementation is a transaction aware index allocator, like we
> > >> >> > have for memory (rocker_port_kzalloc etc...).  But that does seem like
> > >> >> > overkill.
> > >> >> >
> > >> >> > I think that we can make entry->index consistent across
> > >> >> > calls in the same transaction at the expense of breaking the
> > >> >> > rule that per-transaction data should be set during all transaction phases.
> > >> >> >
> > >> >> > Something like this:
> > >> >> >
> > >> >> >
> > >> >> >  if (trans != SWITCHDEV_TRANS_COMMIT)
> > >> >> >          /* Avoid index being set to different values across calls
> > >> >> >           * to this function by the same caller within the same
> > >> >> >           * transaction.
> > >> >> >           */
> > >> >> >          entry->index = rocker->neigh_tbl_next_index++;
> > >> >> >  if (trans == SWITCHDEV_TRANS_PREPARE)
> > >> >> >          return;
> > >> >> >
> > >> >> >
> > >> >>
> > >> >> As long as it is guraded by rtnl lock, no worries about this race?  It
> > >> >> seems to be assumed that prepare-commit is guarded by rtnl lock,
> > >> >> according to commit c4f20321 ("rocker: support prepare-commit
> > >> >> transaction model").
> > >> >>
> > >> >> But as you are concerned, it seems to be able to be called by another
> > >> >> caller, specifically, neigh_timer_handler() in interrupt context without
> > >> >> rtnl lock. IMHO, it should be fixed rather than avoiding the race here.
> > >> >
> > >> >Yes, I believe that is the case I was seeing.
> > >> >
> > >> >Scott, Jiri, how would you like to resolve this?
> > >>
> > >>
> > >> I believe that you can depend on rtnl being held - in switchdev_port_obj_add
> > >> there is ASSERT_RTNL assection at the very beginning of the function.
> > >
> > > In the prepare-commit scenario, yes, I agree that is the case.
> > > But it does not seem to always be the case when the transaction phase is none.
> > >
> > > What I am seeing is:
> > >
> > > 1. rocker_port_ipv4_nh() is called via switchdev_port_obj_add()
> > >    with trans = SWITCHDEV_TRANS_PREPARE
> > >
> > > 2. rocker_port_ipv4_neigh() is called by rocker_neigh_update()
> > >    with trans = SWITCHDEV_TRANS_NONE.
> > >
> > >    The call chain goes up to arp_process() via neigh_update().
> > >
> > > 3. rocker_port_ipv4_nh() is called via switchdev_port_obj_add()
> > >    with trans = SWITCHDEV_TRANS_COMMIT
> > >
> > > I believe #2 is not guarded by rtnl.
> >
> > Looks like rocker->neigh_tbl_next_index was a problem even before the
> > transaction model was introduced, due to no protection for concurrent
> > processes in diff contexts.
> >
> > We'll need to turn the NETEVENT_NEIGH_UPDATE into process context and
> > hold rtnl_lock, similar to what we do in
> > rocker_event_mac_vlan_seen_work().  That, plus Toshiaki's suggested
> > change for _rocker_neigh_add() should do it.
>
> Thanks,
>
> what I suggest is that we modify this patch as per Makita-san's suggestion
> and proceed with it and the rest of the series. And then come back to the
> neigh_tbl_next_index problem.


Agreed.  I'll address the neigh_tbl_next_index problem once your
series goes in.  Thanks Simon and Makita-san.
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