Re: Lockup with 2.6.9-ac15 related to netconsole

From: Francois Romieu
Date: Wed Dec 22 2004 - 07:43:09 EST

Patrick McHardy <kaber@xxxxxxxxx> :
> 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.

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