Re: [PATCH for-next] io_uring: ensure IOSQE_ASYNC file table grabbing works, with SQPOLL

[Jens Axboe <axboe@kernel.dk>]

On 9/10/20 6:37 AM, Pavel Begunkov wrote:
> On 09/09/2020 19:07, Jens Axboe wrote:
>> On 9/9/20 9:48 AM, Pavel Begunkov wrote:
>>> On 09/09/2020 16:10, Jens Axboe wrote:
>>>> On 9/9/20 1:09 AM, Pavel Begunkov wrote:
>>>>> On 09/09/2020 01:54, Jens Axboe wrote:
>>>>>> On 9/8/20 3:22 PM, Jens Axboe wrote:
>>>>>>> On 9/8/20 2:58 PM, Pavel Begunkov wrote:
>>>>>>>> On 08/09/2020 20:48, Jens Axboe wrote:
>>>>>>>>> Fd instantiating commands like IORING_OP_ACCEPT now work with SQPOLL, but
>>>>>>>>> we have an error in grabbing that if IOSQE_ASYNC is set. Ensure we assign
>>>>>>>>> the ring fd/file appropriately so we can defer grab them.
>>>>>>>>
>>>>>>>> IIRC, for fcheck() in io_grab_files() to work it should be under fdget(),
>>>>>>>> that isn't the case with SQPOLL threads. Am I mistaken?
>>>>>>>>
>>>>>>>> And it looks strange that the following snippet will effectively disable
>>>>>>>> such requests.
>>>>>>>>
>>>>>>>> fd = dup(ring_fd)
>>>>>>>> close(ring_fd)
>>>>>>>> ring_fd = fd
>>>>>>>
>>>>>>> Not disagreeing with that, I think my initial posting made it clear
>>>>>>> it was a hack. Just piled it in there for easier testing in terms
>>>>>>> of functionality.
>>>>>>>
>>>>>>> But the next question is how to do this right...> 
>>>>>> Looking at this a bit more, and I don't necessarily think there's a
>>>>>> better option. If you dup+close, then it just won't work. We have no
>>>>>> way of knowing if the 'fd' changed, but we can detect if it was closed
>>>>>> and then we'll end up just EBADF'ing the requests.
>>>>>>
>>>>>> So right now the answer is that we can support this just fine with
>>>>>> SQPOLL, but you better not dup and close the original fd. Which is not
>>>>>> ideal, but better than NOT being able to support it.
>>>>>>
>>>>>> Only other option I see is to to provide an io_uring_register()
>>>>>> command to update the fd/file associated with it. Which may be useful,
>>>>>> it allows a process to indeed to this, if it absolutely has to.
>>>>>
>>>>> Let's put aside such dirty hacks, at least until someone actually
>>>>> needs it. Ideally, for many reasons I'd prefer to get rid of
>>>>
>>>> BUt it is actually needed, otherwise we're even more in a limbo state of
>>>> "SQPOLL works for most things now, just not all". And this isn't that
>>>> hard to make right - on the flush() side, we just need to park/stall the
>>>
>>> I understand that it isn't hard, but I just don't want to expose it to
>>> the userspace, a) because it's a userspace API, so couldn't probably be
>>> killed in the future, b) works around kernel's problems, and so
>>> shouldn't really be exposed to the userspace in normal circumstances.
>>>
>>> And it's not generic enough because of a possible "many fds -> single
>>> file" mapping, and there will be a lot of questions and problems.
>>>
>>> e.g. if a process shares a io_uring with another process, then
>>> dup()+close() would require not only this hook but also additional
>>> inter-process synchronisation. And so on.
>>
>> I think you're blowing this out of proportion. Just to restate the
> 
> I just think that if there is a potentially cleaner solution without
> involving userspace, we should try to look for it first, even if it
> would take more time. That was the point.

Regardless of whether or not we can eliminate that need, at least it'll
be a relaxing of the restriction, not an increase of it. It'll never
hurt to do an extra system call for the case where you're swapping fds.
I do get your point, I just don't think it's a big deal.

>>>>> fcheck(ctx->ring_fd) in favour of synchronisation in io_grab_files(),
>>>>> but I wish I knew how.
>>>>
>>>> That'd be nice, and apply equally to all cases as the SQPOLL case isn't
>>>> special at all anymore.
>>>
>>> I miss the whole story, have you asked fs guys about the problem?
>>> Or is it known that nothing would work?
>>
>> I haven't looked into it.
> 
> Any chance you have someone in mind who can take a look? I don't
> think I have a chance to get to anyone in fs.

I'll take a look.

-- 
Jens Axboe