Re: io_uring file descriptor address already in use error

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe