On Tue, Aug 19, 2008 at 09:16:42AM +0000, Jarek Poplawski wrote:

quoted text
> On Tue, Aug 19, 2008 at 06:55:04PM +1000, Herbert Xu wrote:

...

quoted text
> > In fact we're not really adding anything new here, the qdiscs were

> > not accessed under RCU uniformly.  If you go back in the tree prior

> > to the multi-qdisc stuff, you'll find that only dev_queue_xmit works

> > under RCU.  qdisc_restart does not and therefore deferring the

> > destruction to RCU is pointless anyway.

> >

> > So in fact we've already been relying on the fact that by the time

> > qdisc_destroy comes about nobody on the read side (i.e., the packet

> > transmission path) should have a reference to it.

>

> Let's not discuss using such a qdisc by others but a possibility

> that some common lists could be broken for readers from upper qdiscs.

> (They were not deactivated.) Of course, if it's done properly with

> some references before qdisc_destroy then it's all right. I'd prefer

> to check this later yet.

So, what I was most suspicious of, cls_u32, looks like safe wrt. this.

Congratulations for good estimation of this.
But how about this part in qdisc_destroy():

        if (qdisc->parent)

                list_del(&qdisc->list);
If we do this with child qdisc from qdisc_graft() it's without

deactivation. The rest of the tree can be dequeued in the meantime

and call qdisc_tree_decrease_qlen() (like hfsc, tbf, netem), which

uses qdisc_lookup() to access this list. We list_del() under rtnl

lock only, they lookup under sch_tree_lock(). Is it a bit unsafe

or I miss something?
Thanks,

Jarek P.

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