Messages in this thread | | | Date | Tue, 25 Oct 2016 16:41:26 +0200 | From | Oleg Nesterov <> | Subject | Re: hit a KASan bug related to Perf during stress test |
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On 10/25, Peter Zijlstra wrote: > > 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.
I do not see how can we close this race, we obviously do not want to use any locking.
That is why I tried to suggest
nr = __task_pid_nr_ns(p, type, event->ns); if (!nr && !is_idle_task(p)) nr = -1; return nr;
but this will report -1 if p runs in another namespace, so perhaps we can do
nr = __task_pid_nr_ns(p, type, event->ns); if (!nr && p->exit_state) // it has already called exit_notify nr = -1; return nr;
Oleg.
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