Re: io_uring file descriptor address already in use error

[Josef <josef.grieb@gmail.com>]

On Wed, 26 Aug 2020 at 15:44, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 8/25/20 9:01 PM, Josef wrote:
> >> In order for the patch to be able to move ahead, we'd need to be able
> >> to control this behavior. Right now we rely on the file being there if
> >> we need to repoll, see:
> >>
> >> commit a6ba632d2c249a4390289727c07b8b55eb02a41d
> >> Author: Jens Axboe <axboe@kernel.dk>
> >> Date:   Fri Apr 3 11:10:14 2020 -0600
> >>
> >>     io_uring: retry poll if we got woken with non-matching mask
> >>
> >> If this never happened, we would not need the file at all and we could
> >> make it the default behavior. But don't think that's solvable.
> >>
> >>> is there no other way around to close the file descriptor? Even if I
> >>> remove the poll, it doesn't work
> >>
> >> If you remove the poll it should definitely work, as nobody is holding a
> >> reference to it as you have nothing else in flight. Can you clarify what
> >> you mean here?
> >>
> >> I don't think there's another way, outside of having a poll (io_uring
> >> or poll(2), doesn't matter, the behavior is the same) being triggered in
> >> error. That doesn't happen, as mentioned if you do epoll/poll on a file
> >> and you close it, it won't trigger an event.
> >>
> >>> btw if understood correctly poll remove operation refers to all file
> >>> descriptors which arming a poll in the ring buffer right?
> >>> Is there a way to cancel a specific file descriptor poll?
> >>
> >> You can cancel specific requests by identifying them with their
> >> ->user_data. You can cancel a poll either with POLL_REMOVE or
> >> ASYNC_CANCEL, either one will find it. So as long as you have that, and
> >> it's unique, it'll only cancel that one specific request.
> >
> > thanks it works, my bad, I was not aware that user_data is associated
> > with the poll request user_data...just need to remove my server socket
> > poll which binds to an address so I think this patch is not really
> > necessary
> >
> > btw IORING_FEAT_FAST_POLL feature which arming poll for read events,
> > how does it work when the file descriptor(not readable yet) wants to
> > read(non blocking) something and I close(2) the file descriptor? I'm
> > guessing io_uring doesn't hold any reference to it anymore right?
>
> Most file types will *not* notify you through poll if they get closed,
> so it'll just sit there until canceled. This is the same with poll(2) or
> epoll(2). io_uring will continue to hold a reference to the file, it
> does that over request completion for any request that uses a file.


okay, btw according to the man page IORING_OP_POLL_REMOVE it's unclear
to me what value user_data contains in the cqe
I'm guessing user_data is always 0 right? just tested it with liburing
I got always 0(user_data) even if there is no polling request


---
Josef Grieb