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

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

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
fcheck(ctx->ring_fd) in favour of synchronisation in
io_grab_files(), but I wish I knew how.

-- 
Pavel Begunkov