Problems replacing epoll with io_uring in tevent

[Stefan Metzmacher]

Hi Jens,

>> I'm currently trying to prototype for an IORING_POLL_CANCEL_ON_CLOSE
>> flag that can be passed to POLL_ADD. With that we'll register
>> the request in &req->file->f_uring_poll (similar to the file->f_ep list for epoll)
>> Then we only get a real reference to the file during the call to
>> vfs_poll() otherwise we drop the fget/fput reference and rely on
>> an io_uring_poll_release_file() (similar to eventpoll_release_file())
>> to cancel our registered poll request.
> 
> Yes, this is a bit tricky as we hold the file ref across the operation. I'd
> be interested in seeing your approach to this, and also how it would
> interact with registered files...

It should work fine with fixed files, but I haven't tested it.

But from reading the code I'm wondering what happens in general with pending
requests on a closed fixed file? There's no referencing in io_file_get_fixed(),
or is it done via io_req_set_rsrc_node() together with the
io_rsrc_node_switch_start()/io_queue_rsrc_removal()/io_rsrc_node_switch()
within io_fixed_fd_remove()?

But IORING_POLL_CANCEL_ON_CLOSE doesn't have any effect together with
REQ_F_FIXED_FILE (in my current code). Maybe a io_fixed_fd_remove()
should call __io_async_cancel with IORING_ASYNC_CANCEL_FD_FIXED.

I was also thinking if any pending could be canceled by a close(),
because they all have the registered in the struct file list...
But that might be overkill and io_uring aware applications can just use
IORING_ASYNC_CANCEL_FD_* explicitly. Also fixed files are also only
used by io_uring aware code.

metze