syzbot has bisected this bug to:

commit b636fd38dc40113f853337a7d2a6885ad23b8811
Author: Tejun Heo [off-list ref]
Date:   Fri May 31 17:38:58 2019 +0000

     cgroup: Implement css_task_iter_skip()

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1256fcd2a00000
start commit:   56b697c6 Add linux-next specific files for 20190604
git tree:       linux-next
final crash:    https://syzkaller.appspot.com/x/report.txt?x=1156fcd2a00000
console output: https://syzkaller.appspot.com/x/log.txt?x=1656fcd2a00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4248d6bc70076f7d
dashboard link: https://syzkaller.appspot.com/bug?extid=9343b7623bc03dc680c1
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=102ab292a00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15f0e27ca00000

Reported-by: syzbot+9343b7623bc03dc680c1@syzkaller.appspotmail.com
Fixes: b636fd38dc40 ("cgroup: Implement css_task_iter_skip()")

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