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Subject: Re: [PATCH liburing v2 0/1] test: add epoll test case
From:   Jens Axboe <axboe@kernel.dk>
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     linux-kernel@vger.kernel.org, io-uring@vger.kernel.org
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Date:   Thu, 6 Feb 2020 12:51:03 -0700
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On 2/6/20 12:15 PM, Jens Axboe wrote:
> On 2/6/20 10:33 AM, Stefano Garzarella wrote:
>>
>>
>> On Fri, Jan 31, 2020 at 4:39 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> On 1/31/20 7:29 AM, Stefano Garzarella wrote:
>>>> Hi Jens,
>>>> this is a v2 of the epoll test.
>>>>
>>>> v1 -> v2:
>>>>     - if IORING_FEAT_NODROP is not available, avoid to overflow the CQ
>>>>     - add 2 new tests to test epoll with IORING_FEAT_NODROP
>>>>     - cleanups
>>>>
>>>> There are 4 sub-tests:
>>>>     1. test_epoll
>>>>     2. test_epoll_sqpoll
>>>>     3. test_epoll_nodrop
>>>>     4. test_epoll_sqpoll_nodrop
>>>>
>>>> In the first 2 tests, I try to avoid to queue more requests than we have room
>>>> for in the CQ ring. These work fine, I have no faults.
>>>
>>> Thanks!
>>>
>>>> In the tests 3 and 4, if IORING_FEAT_NODROP is supported, I try to submit as
>>>> much as I can until I get a -EBUSY, but they often fail in this way:
>>>> the submitter manages to submit everything, the receiver receives all the
>>>> submitted bytes, but the cleaner loses completion events (I also tried to put a
>>>> timeout to epoll_wait() in the cleaner to be sure that it is not related to the
>>>> patch that I send some weeks ago, but the situation doesn't change, it's like
>>>> there is still overflow in the CQ).
>>>>
>>>> Next week I'll try to investigate better which is the problem.
>>>
>>> Does it change if you have an io_uring_enter() with GETEVENTS set? I wonder if
>>> you just pruned the CQ ring but didn't flush the internal side.
>>
>> If I do io_uring_enter() with GETEVENTS set and wait_nr = 0 it solves
>> the issue, I think because we call io_cqring_events() that flushes the
>> overflow list.
>>
>> At this point, should we call io_cqring_events() (that flushes the
>> overflow list) in io_uring_poll()?
>> I mean something like this:
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 77f22c3da30f..2769451af89a 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -6301,7 +6301,7 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
>>         if (READ_ONCE(ctx->rings->sq.tail) - ctx->cached_sq_head !=
>>             ctx->rings->sq_ring_entries)
>>                 mask |= EPOLLOUT | EPOLLWRNORM;
>> -       if (READ_ONCE(ctx->rings->cq.head) != ctx->cached_cq_tail)
>> +       if (!io_cqring_events(ctx, false))
>>                 mask |= EPOLLIN | EPOLLRDNORM;
>>
>>         return mask;
> 
> That's not a bad idea, would just have to verify that it is indeed safe
> to always call the flushing variant from there.

Double checked, and it should be fine. We may be invoked with
ctx->uring_lock held, but that's fine.

-- 
Jens Axboe