Re: [PATCH 1/1] io_uring: fix leaks on IOPOLL and CQE_SKIP

[Pavel Begunkov <asml.silence@gmail.com>]

On 4/15/22 23:41, Pavel Begunkov wrote:
> On 4/15/22 23:03, Jens Axboe wrote:
>> On 4/15/22 3:05 PM, Pavel Begunkov wrote:
>>> On 4/12/22 17:46, Jens Axboe wrote:
>>>> On 4/12/22 10:41 AM, Jens Axboe wrote:
>>>>> On 4/12/22 10:24 AM, Pavel Begunkov wrote:
>>>>>> If all completed requests in io_do_iopoll() were marked with
>>>>>> REQ_F_CQE_SKIP, we'll not only skip CQE posting but also
>>>>>> io_free_batch_list() leaking memory and resources.
>>>>>>
>>>>>> Move @nr_events increment before REQ_F_CQE_SKIP check. We'll potentially
>>>>>> return the value greater than the real one, but iopolling will deal with
>>>>>> it and the userspace will re-iopoll if needed. In anyway, I don't think
>>>>>> there are many use cases for REQ_F_CQE_SKIP + IOPOLL.
>>>>>
>>>>> Ah good catch - yes probably not much practical concern, as the lack of
>>>>> ordering for file IO means that CQE_SKIP isn't really useful for that
>>>>> scenario.
>>>>
>>>> One potential snag is with the change we're now doing
>>>> io_cqring_ev_posted_iopoll() even if didn't post an event. Again
>>>> probably not a practical concern, but it is theoretically a violation
>>>> if an eventfd is used.
>>> Looks this didn't get applied. Are you concerned about eventfd?
>>
>> Yep, was hoping to get a reply back, so just deferred it for now.
>>
>>> Is there any good reason why the userspace can't tolerate spurious
>>> eventfd events? Because I don't think we should care this case
>>
>> I always forget the details on that, but we've had cases like this in
>> the past where some applications assume that if they got N eventfd
>> events, then are are also N events in the ring. Which granted is a bit
>> odd, but it does also make some sense. Why would you have more eventfd
>> events posted than events?
> 
> For the same reason why it can get less eventfd events than there are
> CQEs, as for me it's only a communication channel but not a

s/communication/notification/

> replacement for completion events.

-- 
Pavel Begunkov