On 6/9/21 4:47 PM, Pavel Begunkov wrote: > 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 Apparently, they have *_iter but reasonably don't set FMODE_NOWAIT. Similar goes for /dev/null and others, either they don't have *_iter or don't set FMODE_NOWAIT and ignore IOCB_NOWAIT. Bottom line, we need to tinker cdevs that we're interested to be efficient. -- Pavel Begunkov
