Messages in this thread | | | Date | Thu, 26 Jan 2012 17:26:11 +0100 | From | Oleg Nesterov <> | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition |
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On 01/26, Peter Zijlstra wrote: > > So since we never call schedule() the p->on_rq thing will always be > true. This means we don't need to consider all the icky ttwu after that, > it also means the whole thing is inside ->pi_lock. > > So we only have to consider the exact case Yasunori-San illustrated, and > waiting on ->pi_lock is sufficient.
Yes, and this is why I think Yasunori-san's patch should work. Because, to remind, it adds unlock_wait(pi_lock).
> However I think your proposal: > > > for (;;) { > > tsk->state = TASK_DEAD; > > schedule(); > > } > > should equally work, if we hit the race and call schedule() with ->state > = TASK_RUNNING,
Yes, in this case everything is fine, but we can shedule() with TASK_DEAD state. preempt_disable() can't (and shouldn't) prevent deactivate_task().
To simplify, try_to_wake_up() does
spin_lock(pi_lock);
if (!(p->state & state)) goto out;
/* WINDOW */
if (p->on_rq) { ... everything is fine ... }
p->state = TASK_WAKING; ttwu_queue(p, cpu);
And the exiting task does
// but do not sleep ... current->state = TASK_UNINTERRUPTIBLE; // ttwu() checks ->state ... tsk->state = TASK_DEAD; schedule(); -> deactivate_task(); -> tsk->on_rq = 0; -> finish_task_switch();
// ttwu() checks ->on_rq
In theory it can do this all in the WINDOW above. In this case we can wake it up again, after finish_task_switch()-put_task_struct().
No?
Oleg.
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