syzbot has bisected this bug to:

commit 768134d4f48109b90f4248feecbeeb7d684e410c
Author: Jens Axboe [off-list ref]
Date:   Mon Nov 11 03:30:53 2019 +0000

    io_uring: don't do flush cancel under inflight_lock

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=14233ef5e00000
start commit:   1b649e0b Merge git://git.kernel.org/pub/scm/linux/kernel/g..
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=16233ef5e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=12233ef5e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=27392dd2975fd692
dashboard link: https://syzkaller.appspot.com/bug?extid=313d95e8a7a49263f88d
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13850447e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=119a26f5e00000

Reported-by: syzbot+313d95e8a7a49263f88d@syzkaller.appspotmail.com
Fixes: 768134d4f481 ("io_uring: don't do flush cancel under inflight_lock")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection