On Tue, Aug 12, 2008 at 10:11:03PM -0700, David Miller wrote:

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Wed, 13 Aug 2008 14:30:19 +1000

quoted

David Miller [off-list ref] wrote:

quoted

The qdisc pointer traverses to the softirq handler, which can be run
in a process context (via ksoftirqd), and this pointer gets there
via the per-cpu ->output_queue.

Here are two possible solutions:

1) The active way: smp_call_function and forcibly remove the qdiscs
in question from each output_queue.

2) The passive way: Make dev_deactive call yield() until no qdisc's
are on an output_queue.  This assumes there is some sort of dead
flag detection on the output_queue side so it doesn't keep going
forever.

Yes, we'll need some kind of dead flag it seems.

Another thing we can do is, in the yield loop, grabbing the
__QDISC_STATE_RUNNING bit.

But actually, I think Jarek has a point.

The existing loop there in dev_deactivate() should work _iff_ we make
it look at the proper qdisc.

This is another case where I didn't transform things correctly.  The
old code worked on dev->state since that's where we kept what used
to be __LINK_STATE_QDISC_RUNNING.

But now that state is kept in the qdisc itself.  But we just zapped
the active qdisc, so the old one is in ->qdisc_sleeping not ->qdisc.

So, just like one of Jarek's patches, we should simply change
dev_queue->qdisc into dev_queue->qdisc_sleeping and that should
take care of the bulk of the issues.

Shouldn't it?

If we don't change anything in __netif_schedule() I doubt it's enough.
And if this old way of waiting for "outstanding qdisc-less" calls was
really needed we should probably wait for both qdisc and qdisc_sleeping
then.

Hmmm... maybe we have to sample __QDISC_STATE_SCHED too.

We could probably even think of using this flag napi_disable() way.

Jarek P.