netdev - Re: locating the 'tc actions' hook

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Message-ID: <51FCEE59.1030402@mojatatu.com>
Date:	Sat, 03 Aug 2013 07:49:45 -0400
From:	Jamal Hadi Salim <jhs@...atatu.com>
To:	John Fastabend <john.fastabend@...il.com>
CC:	John Fastabend <john.r.fastabend@...el.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tom Herbert <therbert@...gle.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: locating the 'tc actions' hook



On 13-08-02 02:46 PM, John Fastabend wrote:

> Perhaps another incorrect observation but what protects the tc_actions?
>
> Create a series of actions via 'tc actions' which populates the hash
> table protected by hinfo->lock and also rtnetlink is holding the rtnl
> lock.
>
> Add a filter with index hook to get this action graph attached to a
> filters tcf_exts pointer.
>
> Now for what I think is the race, the classifier will call tcf_exts_exec
> which will call tcf_action_exec() and start walking the actions and
> executing them with the qdisc_lock held.
>
> At the same time tcf_action_destroy() may be called via 'tc actions
> delete' which will only hold the rtnl lock via rtnetlink.
>

One runs in user context and the other in softirq context; so it will
work fine.

cheers,
jamal




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