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

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Date:	Wed, 20 May 2015 13:17:36 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	Simon Horman <simon.horman@...ronome.com>
Cc:	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
	Scott Feldman <sfeldma@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next 3/4] rocker: do not make neighbour entry
 changes when preparing transactions

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.


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