On Thu, 2005-04-14 at 18:46 +0300, Tarhon-Onu Victor wrote:
> On Tue, 12 Apr 2005, Tarhon-Onu Victor wrote:
>
> > So the problem should be looked in that changes to the pkt sched API,
> > the patch containing only those changes is at
>
> The bug is in this portion of code from net/sched/sch_generic.c,
> in the qdisc_destroy() function:
>
> ==
> list_for_each_entry(cq, &cql, list)
> list_for_each_entry_safe(q, n, &qdisc->dev->qdisc_list, list)
> if (TC_H_MAJ(q->parent) == TC_H_MAJ(cq->handle)) {
> if (q->ops->cl_ops == NULL)
> list_del_init(&q->list);
> else
> list_move_tail(&q->list, &cql);
> }
> list_for_each_entry_safe(cq, n, &cql, list)
> list_del_init(&cq->list);
> ==
>
> ...and it happens when q->ops->cl_ops is NULL and
> list_del_init(&q->list) is executed.
>
> The stuff from include/linux/list.h looks ok, it seems like one
> of those two iterations (list_for_each_entry() and
> list_for_each_entry_safe()) enters an endless loop when an element is
> removed from the list under some circumstances.
There's a comment above qdisc_destroy that says:
/* Under dev->queue_lock and BH! */
I'm not so sure this is the case. I've included the emails of those
listed as Authors of sch_generic.c and sch_htb.c, hopefully they are the
ones who can help (if not, sorry to bother you).
The list.h is fine, but if another task goes down this list when it
list_del_init is done, there's a chance that the reading task can get to
the deleted item just as it is being deleted, and has pointed itself to
itself. p->next == p. This would go into an infinite loop.
The reason sysrq works is because this doesn't stop interrupts. But put
a local_irq_save around that list and run your test, I bet you won't be
able to do anything, but power off with the big button.
Hope someone can help. I don't know the queue disciplines well enough to
make a proper fix.
-- Steve
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