Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

[Peter Zijlstra]

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?

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.
                         */

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.