Subject: Re: [PATCH] pkt_sched: Destroy gen estimators under rtnl_lock().

From: Jarek Poplawski <[email-masked]>
Date: Thu, 21 Aug 2008 12:19:20 +0000

On Thu, Aug 21, 2008 at 10:01:12PM +1000, Herbert Xu wrote:
> On Thu, Aug 21, 2008 at 11:55:22AM +0000, Jarek Poplawski wrote:
> > 
> > Sure, here is a scenario:
> > 
> > cpu1					cpu2
> > rtnl_lock()
> > qdisc_graft()
> > // parent != NULL
> > ->cops-graft()
> > notify_and_destroy()			qdisc_run()
> > 					spin_lock(root_lock)
> > qdisc_destroy(old)			dequeue_skb()
> > 					tbf_dequeue()
> > 					qdisc_tree_decrease_qlen()	
> > 					qdisc_lookup()
> > //deleting from qdisc_sleeping->list	//walking qdisc_sleeping->list
> > //under rtnl_lock() only		//under qdisc root_lock only
> > list_del(qdisc->list)			list_for_each_entry(txq_root)
> 
> Good catch.  Longer term we should fix it so that it doesn't do
> the silly lookup at run-time.  In fact we'll be getting rid of
> requeue so there will be no need to do this in TBF at all.
> 
> However, for now please create a patch to put a pair of root locks
> around the list_del in qdisc_destroy.

Since qdisc_destroy() is used for destroying root qdisc too, isn't it
better to get this lock back in notify_and_destroy() like this:

if (old) {
	if (parent)
		sch_tree_lock()
	qdisc_destroy()
	if (parent)
		sch_tree_unlock()
}

Thanks,
Jarek P.