[BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

[oleg@redhat.com (Oleg Nesterov)]

On 05/26, Peter Zijlstra wrote:
>
>  static void ttwu_queue(struct task_struct *p, int cpu)
>  {
> @@ -2631,17 +2650,17 @@ try_to_wake_up(struct task_struct *p, un
>  	while (p->on_cpu) {
>  #ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
>  		/*
> -		 * If called from interrupt context we could have landed in the
> -		 * middle of schedule(), in this case we should take care not
> -		 * to spin on ->on_cpu if p is current, since that would
> -		 * deadlock.
> +		 * In case the architecture enables interrupts in
> +		 * context_switch(), we cannot busy wait, since that
> +		 * would lead to live-locks when an interrupt hits and
> +		 * tries to wake up @prev. So bail and do a complete
> +		 * remote wakeup.
>  		 */
> -		if (p == current) {
> -			ttwu_queue(p, cpu);
> +		if (ttwu_activate_remote(p, wake_flags))

Stupid question. Can't we fix this problem if we do

	-	if (p == current)
	+	if (cpu == raw_smp_processor_id())

?

I forgot the rules... but iirc task_cpu(p) can't be changed under us?

Oleg.