Re: [PATCH] io_uring: fix blocking inline submission

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe