netdev - Re: [PATCH] wireless extensions: play with netns

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Date:	Wed, 17 Jun 2009 17:30:40 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Johannes Berg <johannes@...solutions.net>
Cc:	John Linville <linville@...driver.com>,
	Netdev <netdev@...r.kernel.org>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH] wireless extensions: play with netns

Johannes Berg <johannes@...solutions.net> writes:

> On Wed, 2009-06-17 at 16:54 -0700, Eric W. Biederman wrote:
>
>> > Subject: net: make namespace iteration possible under RCU
>> >
>> > We already call rcu_barrier(), so all we need to take
>> > care of is using proper RCU list add/del primitives.
>> 
>> This of course gives you a network namespace that can be found by for_each_net rcu
>> while the per net exit functions are running.  I think that opens up to races
>> that I don't want to think about.
>
> Indeed. Can we move the rcu_barrier() up? Or we could insert a
> synchronize_rcu() (which is sufficient for rcu_read_lock) before the
> exit functions are run?

I don't think we can move the barrier.   But adding an extra synchronize_rcu
should be fine.  We are talking about the slowest of the slow paths here.
It is so slow it even gets it's own kernel thread.

Eric
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