kthread/smpboot: Serialize kthread parking against wakeup

kthread/smpboot:  Serialize kthread parking against wakeup

The control cpu thread which initiates hotplug calls kthread_park()
for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control
thread wakes up the hotplug thread. There is a chance that wakeup
code sees the hotplug thread (running on AP core) in INTERRUPTIBLE
state, but sets its state to RUNNING after hotplug thread has entered
kthread_parkme() and changed its state to TASK_PARKED. This can result
in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED
flag set but fails to rebind the kthread, due to it being not in
TASK_PARKED state. Fix this, by serializing wakeup state change,
against state change before parking the kthread.

Below is the possible race:

Control thread				      Hotplug Thread

kthread_park()
set KTHREAD_SHOULD_PARK
					      smpboot_thread_fn
					      set_current_state(TASK_INTERRUPTIBLE);
					      kthread_parkme

wake_up_process()

raw_spin_lock_irqsave(&p->pi_lock, flags);
if (!(p->state & state)) -> this will fail
            goto out;

					      __kthread_parkme
					       __set_current_state(TASK_PARKED);

if (p->on_rq && ttwu_remote(p, wake_flags))
    ttwu_remote()
        p->state = TASK_RUNNING;
						schedule();

So to avoid this race, take pi_lock to serial state changes.

Change-Id: Ie71645d37046f7ee74df880dbead29efbaad199a
Suggested-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>