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Date:	Thu, 22 Dec 2011 09:42:41 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>
Cc:	Hiroyuki KAMEZAWA <kamezawa.hiroyu@...fujitsu.com>,
	Motohiro Kosaki <kosaki.motohiro@...fujitsu.com>,
	Linux Kernel ML <linux-kernel@...r.kernel.org>
Subject: [BUG] TASK_DEAD task is able to be woken up in special condition


Hello

I found TASK_DEAD task is able to be woken up in special condition.
I would like to report this bug. Please check it.

Here is the sequence how it occurs.

----------------------------------+-----------------------------
                                  |
           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!
--------------------------------------------------------


You probably think that execution time between (*A) and (*B) is very short,
because the interruption is disabled, and setting TASK_RUNNING at (*C)
must be executed before setting TASK_DEAD.


HOWEVER, if SMI is interrupted between (*A) and (*B), 
(*C) is able to execute AFTER setting TASK_DEAD!
Then, exited task is scheduled again, and BUG_ON() is called....

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.


Thanks, 

----
Signed-off-by: Yasunori Goto <y-goto@...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".  */




-- 
Yasunori Goto 


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