Re: Lockup with 2.6.9-ac15 related to netconsole

[Francois Romieu <romieu@fr.zoreil.com>]

Patrick McHardy <kaber@trash.net> :
[...]
> at least the queued messages ordered. But you need to grab
> dev->queue_lock, otherwise you risk corrupting qdisc internal data.
> You should probably also deal with the noqueue-qdisc, which doesn't
> have an enqueue function. So it should look something like this:

If I am not mistaken, a failure on spin_trylock + the test on
xmit_lock_owner imply that it is safe to directly handle the
queue. It means that qdisc_run() has been interrupted on the
current cpu and the other paths seem fine as well. Counter-example
is welcome (no joke).

Of course the patch is completely ugly and violates any layering
principle one could think of. It was not submitted for inclusion :o)

> while (!spin_trylock(&np->dev->xmit_lock)) {
> 	if (np->dev->xmit_lock_owner == smp_processor_id()) {
> 		struct Qdisc *q;
> 
> 		rcu_read_lock();
> 		q = rcu_dereference(dev->qdisc);
> 		if (q->enqueue) {
> 			spin_lock(&dev->queue_lock);

I'd expect it to deadlock if dev_queue_xmit -> qdisc_run is interrupted
on the current cpu and a printk is issued as dev->queue_lock will have
been taken elsewhere.

--
Ueimor