Re: call_rcu from trace_preempt

[Alexei Starovoitov <ast@plumgrid.com>]

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.