[BUG] TASK_DEAD task is able to be woken up in special condition
From: Yasunori Goto
Date: Wed Dec 21 2011 - 19:43:11 EST
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@xxxxxxxxxxxxxx>
---
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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