Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

Kohli, Gaurav
2018-04-24T18:46:28Z

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@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());
> 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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