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

Yasunori Goto
2012-01-17T08:41:23Z

> On Mon, 2012-01-16 at 20:51 +0900, Yasunori Goto wrote:
>
> > try_to_wake_up() has a problem which may change status from TASK_DEAD to
> > TASK_RUNNING in race condition with SMI or guest environment of virtual
> > machine. (See: https://lkml.org/lkml/2011/12/21/523)
> > As a result, exited task is scheduled() again and panic occurs.
> >
> > By this patch, do_exit() waits for releasing task->pi_lock which is used
> > in try_to_wake_up(). It guarantees the task becomes TASK_DEAD after
> > waking up.
>
>
> This Changelog isn't very good. Please spell out the problem instead of
> referring to it so that people using git-blame and the like don't then
> have to go look up some (possibly non-existent) web-resource.

Ahh, Ok.

I rewrote the description and comment in patch.
Please check them.

-----
try_to_wake_up() has a problem which may change status from TASK_DEAD to
TASK_RUNNING in race condition with SMI or guest environment of virtual
machine. As a result, exited task is scheduled() again and panic occurs.

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!
--------------------------------------------------------

The execution time between (*A) and (*B) is usually 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....

If the system works on guest system of virtual machine, the time
between (*A) and (*B) may be also long due to scheduling of hypervisor,
and same phenomenon can occur.

By this patch, do_exit() waits for releasing task->pi_lock which is used
in try_to_wake_up(). It guarantees the task becomes TASK_DEAD after
waking up.

Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>

---
kernel/exit.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

Index: linux-3.2/kernel/exit.c
===================================================================
--- linux-3.2.orig/kernel/exit.c
+++ linux-3.2/kernel/exit.c
@@ -1038,6 +1038,22 @@ NORET_TYPE void do_exit(long code)

preempt_disable();
exit_rcu();
+
+ /*
+ * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
+ * when the following two conditions become true.
+ * - There is race condition of mmap_sem (It is acquired by
+ * exit_mm()), and
+ * - SMI occurs before setting TASK_RUNINNG.
+ * (or hypervisor of virtual machine switches to other guest)
+ * As a result, we may become TASK_RUNNING after becoming TASK_DEAD
+ *
+ * To avoid it, we have to wait for releasing tsk->pi_lock which
+ * is held by try_to_wake_up()
+ */
+ smp_mb();
+ raw_spin_unlock_wait(&tsk->pi_lock);
+
/* causes final put_task_struct in finish_task_switch(). */
tsk->state = TASK_DEAD;
schedule();
-----

--
Yasunori Goto