Messages in this thread | | | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition | From | Peter Zijlstra <> | Date | Wed, 25 Jan 2012 17:51:30 +0100 |
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On Wed, 2012-01-25 at 16:45 +0100, Oleg Nesterov wrote: > On 01/24, Peter Zijlstra wrote: > > > > On Tue, 2012-01-24 at 11:19 +0100, Peter Zijlstra wrote: > > > On Wed, 2012-01-18 at 15:20 +0100, Oleg Nesterov wrote: > > > > do_exit() is different because it can not handle the spurious wakeup. > > > > Well, may be we can? we can simply do > > > > > > > > for (;;) { > > > > tsk->state = TASK_DEAD; > > > > schedule(); > > > > } > > > > > > > > __schedule() can't race with ttwu() once it takes rq->lock. If the > > > > exiting task is deactivated, finish_task_switch() will see EXIT_DEAD. > > > > > > TASK_DEAD, right? > > Yes, but... I simply can't understand what I was thinking about. > And probably I missed something again, but I think this can't work.
Oh man, total confusion. :-) Every time I look at this bug I see different shadows on the wall.
> Afaics, this can only help to prevent the race with ttwu_remote() > doing ttwu_do_wakeup() under rq->lock.
ttwu_do_wakeup() must always be called with rq->lock held.
> But we still can race with the !p->on_rq case which sets TASK_WAKING. > It can do this after finish_task_switch() observes TASK_DEAD and does > put_task_struct().
<random scribbling that got erased>
No, see below !p->on_rq isn't possible and thus pi_lock is indeed sufficient.
> > I think Yasunori-San's patch isn't > > sufficient, note how the p->state = TASK_RUNNING in ttwu_do_wakeup() can > > happen outside of p->pi_lock when the task gets queued on a remote cpu. > > Hmm, really? I am not sure, but I do not trust myself. > > To simplify, you mean that > > mb(); > unlock_wait(pi_lock); > > tsk->state = TASK_DEAD; > > can change ->state from TASK_WAKING to TASK_DEAD, right? Is this really > possible? ttwu() ensures p->on_rq == F in this case.
Ahhh.. hold on, p->on_rq must be true, since we disabled preemption before setting TASK_DEAD, so the thing cannot be scheduled out.
Does this mean that both Yasunori-San's solution and yours work again?
/me goes in search of a fresh mind.. shees!
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