Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil

[Waiman Long <longman@redhat.com>]

On 11/29/2018 10:21 AM, Waiman Long wrote:
> On 11/29/2018 08:12 AM, Peter Zijlstra wrote:
>> +Cc davidlohr and waiman
>>
>> On Thu, Nov 29, 2018 at 08:50:30PM +0800, Yongji Xie wrote:
>>> From: Xie Yongji <xieyongji@baidu.com>
>>>
>>> Our system encountered a problem recently, the khungtaskd detected
>>> some process hang on mmap_sem. But the odd thing was that one task which
>>> is not on mmap_sem.wait_list still sleeps in rwsem_down_read_failed().
>>> Through code inspection, we found a potential bug can lead to this.
>>>
>>> Imaging this:
>>>
>>> Thread 1                                  Thread 2
>>>                                           down_write();
>>> rwsem_down_read_failed()
>>>  raw_spin_lock_irq(&sem->wait_lock);
>>>  list_add_tail(&waiter.list, &wait_list);
>>>  raw_spin_unlock_irq(&sem->wait_lock);
>>>                                           __up_write();
>>>                                            rwsem_wake();
>>>                                             __rwsem_mark_wake();
>>>                                              wake_q_add();
>>>                                              list_del(&waiter->list);
>>>                                              waiter->task = NULL;
>>>  while (true) {
>>>   set_current_state(TASK_UNINTERRUPTIBLE);
>>>   if (!waiter.task) // true
>>>       break;
>>>  }
>>>  __set_current_state(TASK_RUNNING);
>>>
>>> Now Thread 1 is queued in Thread 2's wake_q without sleeping. Then
>>> Thread 1 call rwsem_down_read_failed() again because Thread 3
>>> hold the lock, if Thread 3 tries to queue Thread 1 before Thread 2
>>> do wakeup, it will fail and miss wakeup:
>>>
>>> Thread 1                                  Thread 2      Thread 3
>>>                                                         down_write();
>>> rwsem_down_read_failed()
>>>  raw_spin_lock_irq(&sem->wait_lock);
>>>  list_add_tail(&waiter.list, &wait_list);
>>>  raw_spin_unlock_irq(&sem->wait_lock);
>>>                                                         __rwsem_mark_wake();
>>>                                                          wake_q_add();
>>>                                           wake_up_q();
>>>                                                          waiter->task = NULL;
>>>  while (true) {
>>>   set_current_state(TASK_UNINTERRUPTIBLE);
>>>   if (!waiter.task) // false
>>>       break;
>>>   schedule();
>>>  }
>>>                                                         wake_up_q(&wake_q);
>>>
>>> In another word, that means we might issue the wakeup before setting the reader
>>> waiter to nil. If so, the wakeup may do nothing when it was called before reader
>>> set task state to TASK_UNINTERRUPTIBLE. Then we would have no chance to wake up
>>> the reader any more, and cause other writers such as "ps" command stuck on it.
>>>
>>> This patch is not verified because we still have no way to reproduce the problem.
>>> But I'd like to ask for some comments from community firstly.
>> Urgh; so the case where the cmpxchg() fails because it already has a
>> wakeup in progress, which then 'violates' our expectation of when the
>> wakeup happens.
>>
>> Yes, I think this is real, and worse, I think we need to go audit all
>> wake_q_add() users and document this behaviour.
> Yes, I also think this is a valid race scenario that can cause missed
> wakeup. Actually, I had bug reports of similar symptom of sleeping
> reader not in a wait queue.  I was puzzled by how that could happen.
> That clearly is one possible cause of that.
>
>
>> In the ideal case we'd delay the actual wakeup to the last wake_up_q(),
>> but I don't think we can easily fix that.
>>
>>> Signed-off-by: Xie Yongji <xieyongji@baidu.com>
>>> Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
>>> ---
>>>  kernel/locking/rwsem-xadd.c | 11 +++++++++--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
>>> index 09b1800..50d9af6 100644
>>> --- a/kernel/locking/rwsem-xadd.c
>>> +++ b/kernel/locking/rwsem-xadd.c
>>> @@ -198,15 +198,22 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
>>>  		woken++;
>>>  		tsk = waiter->task;
>>>  
>>> -		wake_q_add(wake_q, tsk);
>>> +		get_task_struct(tsk);
>>>  		list_del(&waiter->list);
>>>  		/*
>>> -		 * Ensure that the last operation is setting the reader
>>> +		 * Ensure calling get_task_struct() before setting the reader
>>>  		 * waiter to nil such that rwsem_down_read_failed() cannot
>>>  		 * race with do_exit() by always holding a reference count
>>>  		 * to the task to wakeup.
>>>  		 */
>>>  		smp_store_release(&waiter->task, NULL);
>>> +		/*
>>> +		 * Ensure issuing the wakeup (either by us or someone else)
>>> +		 * after setting the reader waiter to nil.
>>> +		 */
>>> +		wake_q_add(wake_q, tsk);
>>> +		/* wake_q_add() already take the task ref */
>>> +		put_task_struct(tsk);
>>>  	}
>>>  
>>>  	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
> I doubt putting wake_q_add() after clearing waiter->task can really fix
> the problem. The wake_up_q() function happens asynchronous to the
> detection of NULL waiter->task in __rwsem_down_read_failed_common(). I
> believe the same scenario may still happen.
>
> One possible solution that I can think of is as follows:
>
> diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
> index 10b19a1..1513cdc 100644
> --- a/include/linux/sched/wake_q.h
> +++ b/include/linux/sched/wake_q.h
> @@ -47,6 +47,14 @@ static inline void wake_q_init(struct wake_q_head *head)
>         head->lastp = &head->first;
>  }
>  
> +/*
> + * Return true if the current task is on a wake_q.
> + */
> +static inline bool wake_q_pending(void)
> +{
> +       return !!current->wake_q.next;
> +}
> +
>  extern void wake_q_add(struct wake_q_head *head,
>                        struct task_struct *task);
>  extern void wake_up_q(struct wake_q_head *head);
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 3dbe593..b656777 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -269,7 +269,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
>         /* wait to be given the lock */
>         while (true) {
>                 set_current_state(state);
> -               if (!waiter.task)
> +               if (!smp_load_acquire(&waiter.task))
>                         break;
>                 if (signal_pending_state(state, current)) {
>                         raw_spin_lock_irq(&sem->wait_lock);
> @@ -282,6 +282,15 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
>         }
>  
>         __set_current_state(TASK_RUNNING);
> +
> +       /*
> +        * If waiter is still queuing in a wake_q somewhere, we have to wait
> +        * until the wake_up_q() process is complete as the memory of the
> +        * waiter structure will no longer be valid when we return.
> +        */

Sorry, the comment is wrong. I should say something like
/*
 * If we are still queuing in a wake_q somewhere, we have to wait until
the wake_up_q() function is complete to prevent against concurrent
wake_q operation.
 */
> +       while (wake_q_pending())
> +               cpu_relax();
> +
>         return sem;
>  out_nolock:
>         list_del(&waiter.list);
>
>
Cheers,
Longman