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
