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

Yasunori Goto
2012-01-16T11:52:04Z

> Ok. Thansk for your agreement.
> I'll test this patch.

Hi.

I tested this patch. I've not found any problems so far.
Please merge it.

-----
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.

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

---
kernel/exit.c | 8 ++++++++
1 file changed, 8 insertions(+)

Index: linux-3.2/kernel/exit.c
===================================================================
--- linux-3.2.orig/kernel/exit.c
+++ linux-3.2/kernel/exit.c
@@ -1038,6 +1038,14 @@ NORET_TYPE void do_exit(long code)

preempt_disable();
exit_rcu();
+
+ /*
+ * try_to_wake_up() might be waking me up due to race condition.
+ * Make sure it is finished.
+ */
+ 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