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 21:57:47 +0900
From:	Simon Horman <simon.horman@...ronome.com>
To:	Jiri Pirko <jiri@...nulli.us>
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

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.
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