Re: [PATCH] io_uring: fix blocking inline submission

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

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

-- 
Pavel Begunkov