Messages in this thread Patch in this message | | | Date | Wed, 21 Dec 2011 21:14:14 -0500 | From | KOSAKI Motohiro <> | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition |
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(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); }
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