regression: fixed file hang

[Jens Axboe <axboe@kernel.dk>]

Hi Xiaoguang,

Was doing some other testing today, and noticed a hang with fixed files.
I did a bit of poor mans bisecting, and came up with this one:

commit 0558955373023b08f638c9ede36741b0e4200f58
Author: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Date:   Tue Mar 31 14:05:18 2020 +0800

    io_uring: refactor file register/unregister/update handling

If I revert this one, the test completes fine.

The case case is pretty simple, just run t/io_uring from the fio
repo, default settings:

[ fio] # t/io_uring /dev/nvme0n1p2
Added file /dev/nvme0n1p2
sq_ring ptr = 0x0x7fe1cb81f000
sqes ptr    = 0x0x7fe1cb81d000
cq_ring ptr = 0x0x7fe1cb81b000
polled=1, fixedbufs=1, buffered=0 QD=128, sq_ring=128, cq_ring=256
submitter=345
IOPS=240096, IOS/call=32/31, inflight=91 (91)
IOPS=249696, IOS/call=32/31, inflight=99 (99)
^CExiting on signal 2

and ctrl-c it after a second or so. You'll then notice a kworker that
is stuck in io_sqe_files_unregister(), here:

	/* wait for all refs nodes to complete */
	wait_for_completion(&data->done);

I'll try and debug this a bit, and for some reason it doens't trigger
with the liburing fixed file setup. Just wanted to throw this out there,
so if you have cycles, please do take a look at it.

-- 
Jens Axboe