Re: hit a KASan bug related to Perf during stress test

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 24, 2016 at 05:39:08PM +0200, Oleg Nesterov wrote:
> On 10/24, Peter Zijlstra wrote:
> >
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -1257,7 +1257,14 @@ static u32 perf_event_pid(struct perf_event *event, struct task_struct *p)
> >  	if (event->parent)
> >  		event = event->parent;
> >  
> > -	return task_tgid_nr_ns(p, event->ns);
> > +	/*
> > +	 * It is possible the task already got unhashed, in which case we
> > +	 * cannot determine the current->group_leader/real_parent.
> > +	 *
> > +	 * Also, report -1 to indicate unhashed, so as not to confused with
> > +	 * 0 for the idle task.
> > +	 */
> > +	return pid_alive(p) ? task_tgid_nr_ns(p, event->ns) : ~0;
> >  }
> 
> Yes, but this _looks_ racy unless p == current. I mean, pid_alive() makes
> task_tgid_nr_ns() safe, but task_tgid_nr_ns() still can return zero _if_
> it can race with the exiting task.

So what serialization would close that race? __task_pid_nr_ns() only
seems to use RCU nothing more.