Re: [PATCH RESEND] fs/epoll: fix the edge-triggered mode for nested epoll

[Jason Baron]

On 9/4/19 5:57 AM, Roman Penyaev wrote:
> On 2019-09-03 23:08, Jason Baron wrote:
>> On 9/2/19 11:36 AM, Roman Penyaev wrote:
>>> Hi,
>>>
>>> This is indeed a bug. (quick side note: could you please remove efd[1]
>>> from your test, because it is not related to the reproduction of a
>>> current bug).
>>>
>>> Your patch lacks a good description, what exactly you've fixed. Let
>>> me speak out loud and please correct me if I'm wrong, my understanding
>>> of epoll internals has become a bit rusty: when epoll fds are nested
>>> an attempt to harvest events (ep_scan_ready_list() call) produces a
>>> second (repeated) event from an internal fd up to an external fd:
>>>
>>> ÂÂÂÂ epoll_wait(efd[0], ...):
>>> ÂÂÂÂÂÂ ep_send_events():
>>> ÂÂÂÂÂÂÂÂÂ ep_scan_ready_list(depth=0):
>>> ÂÂÂÂÂÂÂÂÂÂÂ ep_send_events_proc():
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ep_item_poll():
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ep_scan_ready_list(depth=1):
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ep_poll_safewake():
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ep_poll_callback()
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ list_add_tail(&epi, &epi->rdllist);
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ^^^^^^
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ repeated event
>>>
>>>
>>> In your patch you forbid wakeup for the cases, where depth != 0, i.e.
>>> for all nested cases. That seems clear. But what if we can go further
>>> and remove the whole chunk, which seems excessive:
>>>
>>> @@ -885,26 +886,11 @@ static __poll_t ep_scan_ready_list(struct
>>> eventpoll *ep,
>>>
>>> -
>>> -ÂÂÂÂÂÂ if (!list_empty(&ep->rdllist)) {
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Wake up (if active) both the eventpoll wait list and
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * the ->poll() wait list (delayed after we release the
>>> lock).
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (waitqueue_active(&ep->wq))
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ wake_up(&ep->wq);
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (waitqueue_active(&ep->poll_wait))
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pwake++;
>>> -ÂÂÂÂÂÂ }
>>> ÂÂÂÂÂÂÂ write_unlock_irq(&ep->lock);
>>>
>>> ÂÂÂÂÂÂÂ if (!ep_locked)
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ mutex_unlock(&ep->mtx);
>>>
>>> -ÂÂÂÂÂÂ /* We have to call this outside the lock */
>>> -ÂÂÂÂÂÂ if (pwake)
>>> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ep_poll_safewake(&ep->poll_wait);
>>>
>>>
>>> I reason like that: by the time we've reached the point of scanning events
>>> for readiness all wakeups from ep_poll_callback have been already fired and
>>> new events have been already accounted in ready list (ep_poll_callback()
>>> calls
>>> the same ep_poll_safewake()). Here, frankly, I'm not 100% sure and probably
>>> missing some corner cases.
>>>
>>> Thoughts?
>>
>> So the: 'wake_up(&ep->wq);' part, I think is about waking up other
>> threads that may be in waiting in epoll_wait(). For example, there may
>> be multiple threads doing epoll_wait() on the same epoll fd, and the
>> logic above seems to say thread 1 may have processed say N events and
>> now its going to to go off to work those, so let's wake up thread 2 now
>> to handle the next chunk.
> 
> Not quite. Thread which calls ep_scan_ready_list() processes all the
> events, and while processing those, removes them one by one from the
> ready list. But if event mask is !0 and event belongs to
> Level Triggered Mode descriptor (let's say default mode) it tails event
> again back to the list (because we are in level mode, so event should
> be there). So at the end of this traversing loop ready list is likely
> not empty, and if so, wake up again is called for nested epoll fds.
> But, those nested epoll fds should get already all the notifications
> from the main event callback ep_poll_callback(), regardless any thread
> which traverses events.
> 
> I suppose this logic exists for decades, when Davide (the author) was
> reshuffling the code here and there.
> 
> But I do not feel confidence to state that this extra wakeup is bogus,
> I just have a gut feeling that it looks excessive.

Note that I was talking about the wakeup done on ep->wq not ep->poll_wait.
The path that I'm concerned about is let's say that there are N events
queued on the ready list. A thread that was woken up in epoll_wait may
decide to only process say N/2 of then. Then it will call wakeup on ep->wq
and this will wakeup another thread to process the remaining N/2. Without
the wakeup, the original thread isn't going to process the events until
it finishes with the original N/2 and gets back to epoll_wait(). So I'm not
sure how important that path is but I wanted to at least note the change
here would impact that behavior.

Thanks,

-Jason


> 
>> So I think removing all that even for the
>> depth 0 case is going to change some behavior here. So perhaps, it
>> should be removed for all depths except for 0? And if so, it may be
>> better to make 2 patches here to separate these changes.
>>
>> For the nested wakeups, I agree that the extra wakeups seem unnecessary
>> and it may make sense to remove them for all depths. I don't think the
>> nested epoll semantics are particularly well spelled out, and afaict,
>> nested epoll() has behaved this way for quite some time. And the current
>> behavior is not bad in the way that a missing wakeup or false negative
>> would be.
> 
> That's 100% true! For edge mode extra wake up is not a bug, not optimal
> for userspace - yes, but that can't lead to any lost wakeups.
> 
> -- 
> Roman
>