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

KOSAKI Motohiro
2011-12-22T02:14:21Z

(12/21/11 7:42 PM), Yasunori Goto wrote:
>
> 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". */

I doubt it is not only TASK_DEAD issue, it is rwsem fundamental issue.
Because of, a lot of place assume "current->state = newstate" is safe
and don't need any synchronization. So, I'm worry about to lost
TASK_UNINTERRUPTIBLE can make catastrophe like TASK_DEAD.

How about following patch? anyway, rwsem_down_failed_common() is
definitely slowpath. so killing micro optimization is not so much
problem, I guess.

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 410aa11..e2a0c9a 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -208,9 +208,9 @@ rwsem_down_failed_common(struct rw_semaphore *sem,

/* wait to be given the lock */
for (;;) {
+ schedule();
if (!waiter.task)
break;
- schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}