Re: call_rcu from trace_preempt

Steven Rostedt
2015-06-16T17:37:19Z

On Tue, 16 Jun 2015 10:20:05 -0700
Alexei Starovoitov <ast@plumgrid.com> wrote:

> On 6/16/15 5:38 AM, Daniel Wagner wrote:
> > static int free_thread(void *arg)
> > +{
> > + unsigned long flags;
> > + struct htab_elem *l;
> > +
> > + while (!kthread_should_stop()) {
> > + spin_lock_irqsave(&elem_freelist_lock, flags);
> > + while (!list_empty(&elem_freelist)) {
> > + l = list_entry(elem_freelist.next,
> > + struct htab_elem, list);
> > + list_del(&l->list);
> > + kfree(l);
>
> that's not right, since such thread defeats rcu protection of lookup.
> We need either kfree_rcu/call_rcu or synchronize_rcu.
> Obviously the former is preferred that's why I'm still digging into it.
> Probably a thread that does kfree_rcu would be ok, but we shouldn't
> be doing it unconditionally. For all networking programs and 99%
> of tracing programs the existing code is fine and I don't want to
> slow it down to tackle the corner case.
> Extra spin_lock just to add it to the list is also quite costly.

Use a irq_work() handler to do the kfree_rcu(), and use llist (lockless
list) to add items to the list.

-- Steve