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

Yasunori Goto
2011-12-22T00:43:14Z

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@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". */

--
Yasunori Goto