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    SubjectRe: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup
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    On 4/26/2018 1:39 AM, Peter Zijlstra wrote:

    > On Wed, Apr 25, 2018 at 02:03:19PM +0530, Gaurav Kohli wrote:
    >> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
    >> index 5043e74..c5c5184 100644
    >> --- a/kernel/smpboot.c
    >> +++ b/kernel/smpboot.c
    >> @@ -122,7 +122,45 @@ static int smpboot_thread_fn(void *data)
    >> }
    >>
    >> if (kthread_should_park()) {
    >> + /*
    >> + * Serialize against wakeup.
    > *
    > * Prior wakeups must complete and later wakeups
    > * will observe TASK_RUNNING.
    > *
    > * This avoids the case where the TASK_RUNNING
    > * store from ttwu() competes with the
    > * TASK_PARKED store from kthread_parkme().
    > *
    > * If the TASK_PARKED store looses that
    > * competition, kthread_unpark() will go wobbly.
    >> + */
    >> + 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());
    > Does that work for you?

    We have given patch for testing, usually it takes around 2-3 days for reproduction(we will update for the same).

    >
    > But looking at this a bit more; don't we have the exact same problem
    > with the TASK_RUNNING store in the !ht->thread_should_run() case?
    > Suppose a ttwu() happens concurrently there, it can end up competing
    > against the TASK_INTERRUPTIBLE store, no?
    >
    > Of course, that race is not fatal, we'll just end up going around the
    > loop once again I suppose. Maybe a comment there too?
    >
    > /*
    > * A similar race is possible here, but loosing
    > * the TASK_INTERRUPTIBLE store is harmless and
    > * will make us go around the loop once more.
    > */

    Actually instead of race, i am seeing wakeup miss problem which is very rare, if we take case of hotplug thread

    Controller                                           Hotplug

                                                                 Loop start

                                                                 set_current_state(TASK_INTERRUPTIBLE);

                                                                 if (kthread_should_park()) { -> fails

    Set Should_park

    then wake_up

                                                                if (!ht->thread_should_run(td->cpu)) {

                                                                preempt_enable_no_resched();

                                                                schedule(); Again went to schedule(which is very rare to occur,not sure whether it hits)



    >
    > And of course, I suspect we actually want to use TASK_IDLE, smpboot
    > threads don't want signals do they? But that probably ought to be a
    > separate patch.

    Yes I agree, we can control race from here as well,  Please suggest would below change be any help here:

     } else {

                            __set_current_state(TASK_RUNNING);

                            preempt_enable();

                            ht->thread_fn(td->cpu);

                           + set_current_state(TASK_INTERRUPTIBLE);

                           + schedule();

                    }

    >
    --
    Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
    a Linux Foundation Collaborative Project.

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