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Date:	Mon, 31 Dec 2012 10:55:42 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	bhutchings@...arflare.com, faisal.latif@...el.com, fbl@...hat.com,
	roland@...nel.org, sean.hefty@...el.com, hal.rosenstock@...il.com,
	fubar@...ibm.com, andy@...yhouse.net, divy@...lsio.com,
	jitendra.kalsaria@...gic.com, sony.chacko@...gic.com,
	linux-driver@...gic.com, kaber@...sh.net, ursula.braun@...ibm.com,
	blaschka@...ux.vnet.ibm.com, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, ebiederm@...ssion.com, joe@...ches.com,
	amwang@...hat.com, nhorman@...driver.com,
	john.r.fastabend@...el.com, pablo@...filter.org
Subject: Re: [patch net-next 01/15] net: introduce upper device lists

Mon, Dec 31, 2012 at 05:56:33AM CET, shemminger@...tta.com wrote:
>On Sun, 30 Dec 2012 12:58:08 +0100
>Jiri Pirko <jiri@...nulli.us> wrote:
>
>> This lists are supposed to serve for storing pointers to all upper devices.
>> Eventually it will replace dev->master pointer which is used for
>> bonding, bridge, team but it cannot be used for vlan, macvlan where
>> there might be multiple upper present. In case the upper link is
>> replacement for dev->master, it is marked with "master" flag.
>> 
>> New upper device list resolves this limitation. Also, the information
>> stored in lists is used for preventing looping setups like
>> "bond->somethingelse->samebond"
>> 
>> Signed-off-by: Jiri Pirko <jiri@...nulli.us>
>
>What is the use case for this?
>Could you describe a topology that the new upper device list supports,
>that the old scheme does not?


The old scheme is not possible to track the upper-lower dependencies for
all types of devices. (dev->master is used by bonding,bridge,team only).
My new scheme using upper lists allows to track all dependencies. That
provides ability to prevent loops in the dependency trees.


>
>I am concerned that it may open up many new possibilities for user
>error that were not possible before. For example how does it prevent
>an ethernet from being assigned to both a bonding and bridge at the
>same time?

No. netdev_master_upper_dev_link() is used in bond,bridge,team etc.
This function ensures that only one upper device marked as "master" is
present at a time.

This patchset does not introduce any possibility for user error. Only
prevents one group of them - loops.

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