From mboxrd@z Thu Jan 1 00:00:00 1970 From: Simon Horman Subject: Re: [PATCH v3 net-next 3/4] rocker: do not make neighbour entry changes when preparing transactions Date: Wed, 20 May 2015 21:57:47 +0900 Message-ID: <20150520125744.GA2083@penelope.isobedori.kobe.vergenet.net> References: <1432100902-10187-1-git-send-email-simon.horman@netronome.com> <1432100902-10187-4-git-send-email-simon.horman@netronome.com> <555C267B.9070807@lab.ntt.co.jp> <20150520074810.GA21934@vergenet.net> <555C4776.70508@lab.ntt.co.jp> <20150520084623.GA14454@vergenet.net> <20150520111736.GE2228@nanopsycho.orion> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Toshiaki Makita , Scott Feldman , David Miller , netdev@vger.kernel.org To: Jiri Pirko Return-path: Received: from mail-pa0-f51.google.com ([209.85.220.51]:35084 "EHLO mail-pa0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751272AbbETM56 (ORCPT ); Wed, 20 May 2015 08:57:58 -0400 Received: by pacwv17 with SMTP id wv17so66507608pac.2 for ; Wed, 20 May 2015 05:57:57 -0700 (PDT) Content-Disposition: inline In-Reply-To: <20150520111736.GE2228@nanopsycho.orion> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, May 20, 2015 at 01:17:36PM +0200, Jiri Pirko wrote: > Wed, May 20, 2015 at 10:46:26AM CEST, simon.horman@netronome.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.