Messages in this thread | | | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition | From | Peter Zijlstra <> | Date | Fri, 23 Dec 2011 10:49:54 +0100 |
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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?
> 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.
> --- > 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_*().
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