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

Yasunori Goto
2011-12-22T08:22:26Z

Thank you for your response.

> > ----
> > Signed-off-by: Yasunori Goto<y-goto@jp.fujitsu.com>
> >
> > ---
> > kernel/exit.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > Index: linux-3.2-rc4/kernel/exit.c
> > ===================================================================
> > --- linux-3.2-rc4.orig/kernel/exit.c
> > +++ linux-3.2-rc4/kernel/exit.c
> > @@ -1038,8 +1038,11 @@ NORET_TYPE void do_exit(long code)
> >
> > preempt_disable();
> > exit_rcu();
> > +
> > + spin_lock(&tsk->pi_lock, flags);
> > /* causes final put_task_struct in finish_task_switch(). */
> > tsk->state = TASK_DEAD;
> > + spin_unlock(&tsk->pi_lock, flags);
> > schedule();
> > BUG();
> > /* Avoid "noreturn function does return". */
>
> I doubt it is not only TASK_DEAD issue, it is rwsem fundamental issue.
> Because of, a lot of place assume "current->state = newstate" is safe
> and don't need any synchronization. So, I'm worry about to lost
> TASK_UNINTERRUPTIBLE can make catastrophe like TASK_DEAD.

I don't understand why this is catastrophe.
I suppose it is just waken up from TASK_UNINTERRUPTIBLE
by try_to_wake_up() in race condition. It seems to be normal situation.....

But TASK_DEAD status is special. It must not return to TASK_RUNNING state.

>
> How about following patch? anyway, rwsem_down_failed_common() is
> definitely slowpath. so killing micro optimization is not so much
> problem, I guess.
>
>
>
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index 410aa11..e2a0c9a 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -208,9 +208,9 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
>
> /* wait to be given the lock */
> for (;;) {
> + schedule();
> if (!waiter.task)
> break;
> - schedule();
> set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> }
>

Hmmmmmmm.
Are you sure there is no route which TASK_DEAD task is waken up like rwsem?

Thanks.
--
Yasunori Goto