Re: [BUG] TASK_DEAD task is able to be woken up in special condition

Oleg Nesterov
2012-01-06T13:54:42Z

On 01/06, Yasunori Goto wrote:
>
> +change_task_state_cmpxchg(struct task_struct *p, unsigned int target_state,
> + unsigned int new)
> {
> - trace_sched_wakeup(p, true);
> - check_preempt_curr(rq, p, wake_flags);
> + unsigned int old, tmp;
>
> - p->state = TASK_RUNNING;
> + old = p->state;
> +
> + while (1) {
> + if (old == new)
> + return 1;
> +
> + if (unlikely(!(old & target_state)))
> + return 0;
> +
> + tmp = cmpxchg(&p->state, old, new);
> + if (likely(tmp == old))
> + return 1;
> +
> + old = tmp;
> + }

I do not really think we should retry if cmpxchg fails. The state was
changed after initial check, we can pretend it was changed after we
change it succesfully.

> @@ -2828,11 +2883,18 @@ try_to_wake_up(struct task_struct *p, un
> if (!(p->state & state))
> goto out;
>
> - success = 1; /* we're going to change ->state */
> cpu = task_cpu(p);
>
> - if (p->on_rq && ttwu_remote(p, wake_flags))
> - goto stat;
> + if (p->on_rq) {
> + int ret;
> + ret = ttwu_remote(p, state, wake_flags);
> + if (likely(ret == 1)) {
> + success = 1;
> + goto stat;
> + } else if (unlikely(ret < 0))
> + goto out;
> + }
> + success = 1; /* we're going to change ->state */

But this is not enough, you should also recheck the state below. Although
in this case you do not need atomic ops, the target can do nothing with
its ->state.

Anyway. I have to agree with Peter even if I suggested this initially.
This adds the unpleasant complications. Hopefully you found the only
case when the spurious wakeup really hurts, probably it is better to
fix do_exit().

Oleg.