syzbot has bisected this issue to:

commit 24369c2e3bb06d8c4e71fd6ceaf4f8a01ae79b7c
Author: Pavel Begunkov [off-list ref]
Date:   Tue Jan 28 00:15:48 2020 +0000

    io_uring: add io-wq workqueue sharing

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=17934777d00000
start commit:   f88cd3fb Merge tag 'vfio-v5.13-rc5' of git://github.com/aw..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=14534777d00000
console output: https://syzkaller.appspot.com/x/log.txt?x=10534777d00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=82d85e75046e5e64
dashboard link: https://syzkaller.appspot.com/bug?extid=ea2f1484cffe5109dc10
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16d5772fd00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10525947d00000

Reported-by: syzbot+ea2f1484cffe5109dc10@syzkaller.appspotmail.com
Fixes: 24369c2e3bb0 ("io_uring: add io-wq workqueue sharing")

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