netdev - Re: [PATCH net-next v2] rocker: move netevent neigh update to processes context

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Message-ID: <20150604082045.GA7709@vergenet.net>
Date:	Thu, 4 Jun 2015 17:20:48 +0900
From:	Simon Horman <simon.horman@...ronome.com>
To:	David Miller <davem@...emloft.net>
Cc:	sfeldma@...il.com, netdev@...r.kernel.org, jiri@...nulli.us,
	makita.toshiaki@....ntt.co.jp
Subject: Re: [PATCH net-next v2] rocker: move netevent neigh update to
 processes context

Hi Dave,

On Wed, Jun 03, 2015 at 11:38:29PM -0700, David Miller wrote:
> From: sfeldma@...il.com
> Date: Tue,  2 Jun 2015 20:43:28 -0700
> 
> > From: Scott Feldman <sfeldma@...il.com>
> > 
> > v2:
> > 
> > Changes based on review:
> > 
> > - David Miller <davem@...emloft.net> raise problem with system_wq not
> >   preserving queue order to execution order.  To fix, use driver-private
> >   ordered workqueue to preserve ordering of queued work.
> > 
> > - Jiri Pirko <jiri@...nulli.us> small change on kfree of work queue item.
> > 
> > v1:
> > 
> > In review of Simon's patchset "rocker: transaction fixes". it was noted
> > that rocker->neigh_tbl_next_index was unprotected in the call path below
> > and could race with other contexts calling rocker_port_ipv4_neigh():
> 
> How it rocker->neigh_tbl_next_index not protected?
> 
> rocker->neigh_tbl_lock is _always_ held when it is accessed.
> 
> This patch, therefore, looks like completely unnecessary complexity
> to me.  Furthermore, I would completely prefer if the operation stays
> completely synchronous to the call path where the neigh operation
> occurs rather than throwing it out to a workqueue.

What I was seeing is as follows:

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

#1 and #2 are guarded by rtl across those calls but
#2 is not guarded by rtnl.

Inside both rocker_port_ipv4_nh() and rocker_port_ipv4_neigh()
neigh_tbl_lock lock is taken but it is not held across the
two calls to rocker_port_ipv4_nh within a single prepare->commit transaction.

I can double check that the above still occurs, but I'm not aware of any
recent changes that would cause it not to occur any more.

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