netdev - Re: [RFC PATCH net-next v3 1/4] net core: Add IFF_PROTO

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Date:	Wed, 29 Apr 2015 16:07:58 -0700
From:	Anuradha Karuppiah <anuradhak@...ulusnetworks.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Scott Feldman <sfeldma@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Roopa Prabhu <roopa@...ulusnetworks.com>,
	Andy Gospodarek <gospo@...ulusnetworks.com>,
	Wilson Kok <wkok@...ulusnetworks.com>
Subject: Re: [RFC PATCH net-next v3 1/4] net core: Add IFF_PROTO_DOWN support.

On Wed, Apr 29, 2015 at 3:13 PM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
> On Mon, 27 Apr 2015 10:38:21 -0700
> anuradhak@...ulusnetworks.com wrote:
>
>> From: Anuradha Karuppiah <anuradhak@...ulusnetworks.com>
>>
>> This patch introduces an IFF_PROTO_DOWN flag that can be used by
>> user space applications to notify drivers that errors have been
>> detected on the device.
>>
>> Signed-off-by: Anuradha Karuppiah <anuradhak@...ulusnetworks.com>
>> Signed-off-by: Andy Gospodarek <gospo@...ulusnetworks.com>
>> Signed-off-by: Roopa Prabhu <roopa@...ulusnetworks.com>
>> Signed-off-by: Wilson Kok <wkok@...ulusnetworks.com>
>
> I worry that adding another bit to an already complex state API
> will break userspace.
>
> There are lots of things besides iproute2 which look at those
> flags including routing daemons (quagga), network manager, netplugd,
> and switch controllers.

Yes, I understand your concerns here. And tried to work around introducing
a separate error flag by clearing IFF_UP on proto_down/detecting errors (as
Scott also brought up earlier).

That implementation failed because of the following reasons -
1. There is no way to disambiguate between admin_down (!IFF_UP) and an
APP/driver enforced error_down (IFF_PROTO_DOWN). Administrator or
automation-scripts that monitor the config assumed that switch-port
configuration had somehow fallen out of sync (and attempted to reinstate the
admin_up repeatedly).

2. Automatic error recovery was not possible; consider the following scenario
for e.g.
   a. The MLAG peer-link is down so the MLAG app on the secondary switch has
      proto_down’ed all the MLAG ports (including switch-port swp1) by clearing
      IFF_UP.
   b. At the same time the administrator is in the process of making some
      changes on the network connected to swp1. To avoid doing it live he would
      admin_disable swp1 (!IFF_UP) by doing an "ip link set swp1 down" (this
      is a no-op as event #a has already cleared IFF_UP on swp1).
   c. If the MLAG peer-link recovers at this point the MLAG app on the
      secondary switch would try to automatically recover the MLAG ports
      by clearing proto_down (i.e. setting IFF_UP); including on swp1. Doing
      that overrides the administrator’s directive to keep swp1 admin_down.
      Overriding an admin-down in a live network can be very dangerous so it
      is not possible to do auto-error-recovery unless we have a way to
      disambiguate between the admin and error states.
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