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

Yasunori Goto
2011-12-26T06:52:29Z

> On Thu, 2011-12-22 at 09:42 +0900, Yasunori Goto wrote:
> > I found TASK_DEAD task is able to be woken up in special condition.
> > I would like to report this bug. Please check it.
>
> How did you find it? Manual inspection? Inspection of a core-dump?

Original trouble occurred on a distributor's kernel which is based on 2.6.32.
Kernel called BUG() because TASK_DEAD task was scheduled again.
I chased it with trace in crash dump, and I confirmed this sequence.

In my code review, current 3.2-rc6 seems to have same problem, so I posted
this report.

>
> > 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!
> > --------------------------------------------------------
>
> <snip>
>
> > 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.
>
> I don't think this can actually happen, note the raw_spin_unlock_wait()
> in do_exit() long before setting TASK_DEAD, that should synchronize
> against the in-progress wakeup and ensure its finished and has set
> TASK_RUNNING. Spurious wakeups after that won't see a state to act on
> and will terminate immediately without touching state.

As Oleg-san said, raw_spin_unlock_wait() is called before exit_mm().
This race condition occurred after using rwsem of mmap_sem in exit_mm().

Thanks.

>
>
> > ---
> > 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". */
>
> Note, ->pi_lock is a raw_spinlock_t, those should've been raw_spin_*().

--
Yasunori Goto