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

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

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
replacement for completion events.

Ok, we don't want to break old applications, but it's a new most
probably not widely used feature, and we can say that the userspace
has to handle spurious eventfd.


> So while I don't think it's a huge issue, and particularly because
> IOPOLL and eventfd would be a nonsensical combo, it would still be nice
> to generally make sure it's the case.
> 
> This isn't the only one though, so maybe we just apply this fix and do
> a full check down the line. Can't see this one making issues.
> 

-- 
Pavel Begunkov