BUG: HANG_DETECT waiting for migration_cpu_stop() complete

[Peter Zijlstra]

On Tue, Sep 06, 2022 at 04:40:03PM -0400, Waiman Long wrote:

I've not followed the earlier stuff due to being unreadable; just
reacting to this..

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 838623b68031..5d9ea1553ec0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2794,9 +2794,9 @@ static int __set_cpus_allowed_ptr_locked(struct
> task_struct *p,
>                 if (cpumask_equal(&p->cpus_mask, new_mask))
>                         goto out;
> 
> -               if (WARN_ON_ONCE(p == current &&
> -                                is_migration_disabled(p) &&
> -                                !cpumask_test_cpu(task_cpu(p), new_mask)))
> {
> +               if (is_migration_disabled(p) &&
> +                   !cpumask_test_cpu(task_cpu(p), new_mask)) {
> +                       WARN_ON_ONCE(p == current);
>                         ret = -EBUSY;
>                         goto out;
>                 }
> @@ -2818,7 +2818,11 @@ static int __set_cpus_allowed_ptr_locked(struct
> task_struct *p,
>         if (flags & SCA_USER)
>                 user_mask = clear_user_cpus_ptr(p);
> 
> -       ret = affine_move_task(rq, p, rf, dest_cpu, flags);
> +       if (!is_migration_disabled(p) || (flags & SCA_MIGRATE_ENABLE)) {
> +               ret = affine_move_task(rq, p, rf, dest_cpu, flags);
> +       } else {
> +               task_rq_unlock(rq, p, rf);
> +       }

This cannot be right. There might be previous set_cpus_allowed_ptr()
callers that are blocked and waiting for the task to land on a valid
CPU.