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

Oleg Nesterov
2011-05-26T17:51:17Z

On 05/26, Peter Zijlstra wrote:
>
> It has the extra cpu == smp_processor_id() check, but I'm not sure this
> whole case is worth the trouble.

Agreed, this case is very unlikely. Perhaps it makes the code more clear
though, up to you.

But, if we keep this check,

> @@ -2636,9 +2636,17 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> * to spin on ->on_cpu if p is current, since that would
> * deadlock.
> */
> - if (p == current) {
> - ttwu_queue(p, cpu);
> - goto stat;
> + if (cpu == smp_processor_id()) {
> + struct rq *rq;
> +
> + rq = __task_rq_lock(p);
> + if (p->on_cpu) {
> + ttwu_activate(rq, p, ENQUEUE_WAKEUP);
> + ttwu_do_wakeup(rq, p, wake_flags);
> + __task_rq_unlock(rq);

then why we re-check ->on_cpu? Just curious.

Oleg.