Re: [PATCH] io_uring: fix blocking inline submission

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe