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

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Message-ID: <555C4776.70508@lab.ntt.co.jp>
Date:	Wed, 20 May 2015 17:36:06 +0900
From:	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:	Simon Horman <simon.horman@...ronome.com>
CC:	Scott Feldman <sfeldma@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	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

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.

Thanks,
Toshiaki Makita

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