linux-kernel - Re: call_rcu from trace

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Date:	Tue, 16 Jun 2015 13:37:09 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Daniel Wagner <wagi@...om.org>, paulmck@...ux.vnet.ibm.com,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: call_rcu from trace_preempt

On Tue, 16 Jun 2015 10:20:05 -0700
Alexei Starovoitov <ast@...mgrid.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
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