netdev - Re: [PATCH v3 net-next 3/4] rocker: do not make neighbour entry changes when preparing transactions

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Message-ID: <CAE4R7bDY4vNOUjRQQ=CzPRbVKEq4qZh2BVutajrRt3_JsdWdvw@mail.gmail.com>
Date:	Wed, 20 May 2015 10:37:52 -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 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.

-scott
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