On 9/10/20 3:01 PM, Jens Axboe wrote:
> On 9/10/20 12:18 PM, Jens Axboe wrote:
>> 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.
> 
> Sort of half assed attempt...
> 
> Idea is to assign a ->files sequence before we grab files, and then
> compare with the current one once we need to use the files. If they
> mismatch, we -ECANCELED the request.
> 
> For SQPOLL, don't grab ->files upfront, grab a reference to the task
> instead. Use the task reference to assign files when we need it.
> 
> Adding Jann to help poke holes in this scheme. I'd be surprised if it's
> solid as-is, but hopefully we can build on this idea and get rid of the
> fcheck().

Split it into two, to make it easier to reason about. Added a few
comments, etc.

-- 
Jens Axboe