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    SubjectRe: hit a KASan bug related to Perf during stress test
    On 10/26, Peter Zijlstra wrote:
    >
    > On Tue, Oct 25, 2016 at 04:41:26PM +0200, Oleg Nesterov wrote:
    > > >
    > > > 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;
    >
    > I think I'm asking how __task_pid_nr_ns() isn't susceptible to this race
    > ;-)

    which race ? ;) it seems that I confused you. Lets ignore the original
    problem with perf_event_pid()->task_tgid_nr_ns() which can access the
    freed memory. Lets suppose it is already fixed.

    Another problem, as you noted, is that task_tgid_nr_ns/task_pid_nr_ns
    returns zero if the task exits and this zero can be confused with the
    swapper's pid.

    return pid_alive(p) ? task_pid_nr_ns(p, event->ns) : ~0

    still can return zero because pid_alive(p) == T is not stable if we can
    race with the exiting task, so it can't guarantee that task_pid_nr_ns()
    won't return 0.

    So we can check ->exit_state or, even better, that same pid_alive() after
    task_pid_nr_ns() returns 0.

    nr = task_pid_nr_ns(p);
    /* avoid -1 if it is idle thread or runs in another ns */
    if (!nr && !pid_alive(p))
    nr = -1;
    return nr;

    Or I misunderstood you?

    Oleg.

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