Re: [PATCH] kthread: Prevent unpark race which puts threads on the wrong cpu

[Dave Hansen <dave@sr71.net>]

On 04/11/2013 03:19 AM, Thomas Gleixner wrote:
> --- linux-2.6.orig/kernel/smpboot.c
> +++ linux-2.6/kernel/smpboot.c
> @@ -185,8 +185,18 @@ __smpboot_create_thread(struct smp_hotpl
>  	}
>  	get_task_struct(tsk);
>  	*per_cpu_ptr(ht->store, cpu) = tsk;
> -	if (ht->create)
> -		ht->create(cpu);
> +	if (ht->create) {
> +		/*
> +		 * Make sure that the task has actually scheduled out
> +		 * into park position, before calling the create
> +		 * callback. At least the migration thread callback
> +		 * requires that the task is off the runqueue.
> +		 */
> +		if (!wait_task_inactive(tsk, TASK_PARKED))
> +			WARN_ON(1);
> +		else
> +			ht->create(cpu);
> +	}
>  	return 0;
>  }

This one appears to be doing the trick.  I'll run the cpus in an
online/offline loop for a bit and make sure it's stable.  It's passed
several round so far, which is way more than it's done up to this point,
though.