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

[Jens Axboe <axboe@kernel.dk>]

On 9/10/20 7:11 AM, Jens Axboe wrote:
> 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.

BTW, I don't see how we can ever get rid of a need to enter the kernel,
we'd need some chance at grabbing the updated ->files, for instance.
Might be possible to hold a reference to the task and grab it from
there, though feels a bit iffy to hold a task reference from the ring on
the task that holds a reference to the ring. Haven't looked too close,
should work though as this won't hold a file/files reference, it's just
a freeing reference.

-- 
Jens Axboe