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

From: Simon Horman <[email-masked]>
Date: Wed, 20 May 2015 17:46:26 +0900

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?