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From:   Gaurav Kohli <gkohli@codeaurora.org>
To:     tglx@linutronix.de, mpe@ellerman.id.au, dzickus@redhat.com,
        peterz@infradead.org, mingo@kernel.org, bigeasy@linutronix.de
Cc:     linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        Gaurav Kohli <gkohli@codeaurora.org>,
        Neeraj Upadhyay <neeraju@codeaurora.org>
Subject: [PATCH] kthread/smpboot:  Serialize kthread parking against wakeup
Date:   Tue, 24 Apr 2018 14:58:25 +0530
Message-Id: <1524562105-31026-1-git-send-email-gkohli@codeaurora.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

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

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 1650578..514b232 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data)
 		}
 
 		if (kthread_should_park()) {
+			raw_spin_lock(&current->pi_lock);
 			__set_current_state(TASK_RUNNING);
+			raw_spin_unlock(&current->pi_lock);
 			preempt_enable();
 			if (ht->park && td->status == HP_THREAD_ACTIVE) {
 				BUG_ON(td->cpu != smp_processor_id());
-- 
1.9.1