Re: [PATCH] io_uring: fix blocking inline submission

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

On 6/9/21 4:43 PM, Jens Axboe wrote:
> On 6/9/21 9:41 AM, Pavel Begunkov wrote:
>> On 6/9/21 4:36 PM, Jens Axboe wrote:
>>> On 6/9/21 9:34 AM, Pavel Begunkov wrote:
>>>> On 6/9/21 4:07 PM, Jens Axboe wrote:
>>>>> On 6/9/21 5:07 AM, Pavel Begunkov wrote:
>>>>>> There is a complaint against sys_io_uring_enter() blocking if it submits
>>>>>> stdin reads. The problem is in __io_file_supports_async(), which
>>>>>> sees that it's a cdev and allows it to be processed inline.
>>>>>>
>>>>>> Punt char devices using generic rules of io_file_supports_async(),
>>>>>> including checking for presence of *_iter() versions of rw callbacks.
>>>>>> Apparently, it will affect most of cdevs with some exceptions like
>>>>>> null and zero devices.
>>>>>
>>>>> I don't like this, we really should fix the file types, they are
>>>>> broken if they don't honor IOCB_NOWAIT and have ->read_iter() (or
>>>>> the write equiv).
>>>>>
>>>>> For cases where there is no iter variant of the read/write handlers,
>>>>> then yes we should not return true from __io_file_supports_async().
>>>>
>>>> I'm confused. The patch doesn't punt them unconditionally, but make
>>>> it go through the generic path of __io_file_supports_async()
>>>> including checks for read_iter/write_iter. So if a chrdev has
>>>> *_iter() it should continue to work as before.
>>>
>>> Ah ok, yes then that is indeed fine.
>>>
>>>> It fixes the symptom that means the change punts it async, and so
>>>> I assume tty doesn't have _iter()s for some reason. Will take a
>>>> look at the tty driver soon to stop blind guessing.
>>>
>>> I think they do, but they don't honor IOCB_NOWAIT for example. I'd
>>> be curious if the patch actually fixes the reported case, even though
>>> it is most likely the right thing to do. If not, then the fops handler
>>> need fixing for that driver.
>>
>> Yep, weird, but fixes it for me. A simple repro was attached
>> to the issue.
>>
>> https://github.com/axboe/liburing/issues/354
> 
> Ah ok, all good then for now. I have applied the patch, and added
> the reported-by as well.

Great, thanks. Will check what's with the tty part

-- 
Pavel Begunkov