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

Oleg Nesterov
2011-12-23T15:47:27Z

On 12/23, Peter Zijlstra wrote:
>
> On Thu, 2011-12-22 at 09:42 +0900, Yasunori Goto wrote:
> >
> > ----------------------------------+-----------------------------
> > |
> > CPU A | CPU B
> > ----------------------------------+-----------------------------
> > TASK A calls exit()....
> >
> > do_exit()
> >
> > exit_mm()
> > down_read(mm->mmap_sem);
> >
> > rwsem_down_failed_common()
> >
> > set TASK_UNINTERRUPTIBLE
> > set waiter.task <= task A
> > list_add to sem->wait_list
> > :
> > raw_spin_unlock_irq()
> > (I/O interruption occured)
> >
> > __rwsem_do_wake(mmap_sem)
> >
> > list_del(&waiter->list);
> > waiter->task = NULL
> > wake_up_process(task A)
> > try_to_wake_up()
> > (task is still
> > TASK_UNINTERRUPTIBLE)
> > p->on_rq is still 1.)
> >
> > ttwu_do_wakeup()
> > (*A)
> > :
> > (I/O interruption handler finished)
> >
> > if (!waiter.task)
> > schedule() is not called
> > due to waiter.task is NULL.
> >
> > tsk->state = TASK_RUNNING
> >
> > :
> > check_preempt_curr();
> > :
> > task->state = TASK_DEAD
> > (*B)
> > <--- set TASK_RUNNING (*C)
> >
> >
> >
> > schedule()
> > (exit task is running again)
> > BUG_ON() is called!
> > --------------------------------------------------------
>
> <snip>
>
> > This is very bad senario.
> > But, I suppose this phenomenon is able to occur on a guest system of
> > virtual machine too.
> >
> > Please fix it.
> >
> > I suppose task->pi_lock should be held when task->state is changed to
> > TASK_DEAD like the following patch (not tested yet).
> > Because try_to_wake_up() hold it before checking task state.
>
> I don't think this can actually happen, note the raw_spin_unlock_wait()
> in do_exit() long before setting TASK_DEAD, that should synchronize
> against the in-progress wakeup and ensure its finished and has set
> TASK_RUNNING.

How? raw_spin_unlock_wait() is calles before exit_mm(), then the exiting
task plays with its ->state.

IIRC, this was already discussed a bit. Say, try_to_wake_up(TASK_INTERRUPTIBLE)
can wakeup a TASK_UNINTERRUPTIBLE task if it temporary sets INTERRUPTIBLE but
doesn't call schedule() in this state.

May be ttwu_do_wakeup() should do cmpxchg(old_state, RUNNING) ?

Oleg.