syzbot has bisected this bug to:

commit fb041bb7c0a918b95c6889fc965cdc4a75b4c0ca
Author: Will Deacon [off-list ref]
Date:   Thu Nov 21 11:59:00 2019 +0000

    locking/refcount: Consolidate implementations of refcount_t

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=117e9e91e00000
start commit:   2c523b34 Linux 5.6-rc5
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=137e9e91e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=157e9e91e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a5295e161cd85b82
dashboard link: https://syzkaller.appspot.com/bug?extid=cea71eec5d6de256d54d
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=164b5181e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=166dd70de00000

Reported-by: syzbot+cea71eec5d6de256d54d@syzkaller.appspotmail.com
Fixes: fb041bb7c0a9 ("locking/refcount: Consolidate implementations of refcount_t")

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