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Date: Mon, 26 Dec 2011 17:23:58 +0900
From: Yasunori Goto <y-goto@jp.fujitsu.com>
To: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [BUG] TASK_DEAD task is able to be woken up in special condition
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        Hiroyuki KAMEZAWA <kamezawa.hiroyu@jp.fujitsu.com>,
        Motohiro Kosaki <kosaki.motohiro@jp.fujitsu.com>,
        Linux Kernel ML <linux-kernel@vger.kernel.org>
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> 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.

Oleg-san,

Could you point the discussion? 
I don't understand yet how it occurred...


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

-- 
Yasunori Goto