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.
