netdev - dev_change_net_namespace() and synchronize

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Message-ID: <497EE08F.5050704@dev.6wind.com>
Date:	Tue, 27 Jan 2009 11:23:11 +0100
From:	Nicolas Dichtel <nicolas.dichtel@....6wind.com>
To:	netdev <netdev@...r.kernel.org>
Subject: dev_change_net_namespace() and synchronize_net()

Hi folks,

I'm asking me philosophical questions about synchronize_net(). In my 
understanding, it is used to ensure that processing of current packets are finished.

In function dev_change_net_namespace(), synchronize_net() is called twice:

One time, between dev_close() and dev_shutdown(). I don't understand why it's 
needed here:
  - if interface is down, no packet can be processed by it
  - if interface is up, dev_close() will call dev_deactivate() which will call 
synchronize_rcu() (equivalent to synchronize_net()).

Second time, synchronize_net() is called just before exiting. Is it really 
necessary?

Thank you for your answers,
Nicolas
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