On 4/24/2018 11:56 PM, Peter Zijlstra wrote:
On Tue, Apr 24, 2018 at 02:58:25PM +0530, Gaurav Kohli wrote: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@xxxxxxxxxxxxxx> Co-developed-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> Signed-off-by: Gaurav Kohli <gkohli@xxxxxxxxxxxxxx> 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(¤t->pi_lock); __set_current_state(TASK_RUNNING); + raw_spin_unlock(¤t->pi_lock); preempt_enable(); if (ht->park && td->status == HP_THREAD_ACTIVE) { BUG_ON(td->cpu != smp_processor_id());Note how in your scenario above you didn't actually need the TASK_RUNNING state; so how is this change going to fix anything?
Hi Peter, As with help of this , if kthread_should_park run first so wake_up call of controller get exited as task is already set as running, otherwise if controller runs first then we will block here and set running and then sets TASK_PARKED . So no chance of cpuhp set as running during kthread_parkme call. But as we discussed this can be fix by 2nd patch as well, So once you get time and able to see , Please let us know or do you want me to try your 2nd patch for testing first?
But yes, I suspect it is right, but it definitely needs a comment explaining wth we take that lock there. Like I said earlier, my brain is entirely fried for the day; but I'll have a try tomorrow.
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