syzbot has bisected this bug to:

commit 1cf8dfe8a661f0462925df943140e9f6d1ea5233
Author: Peter Zijlstra [off-list ref]
Date:   Sat Jul 13 09:21:25 2019 +0000

     perf/core: Fix race between close() and fork()

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1523f40c600000
start commit:   c6dd78fc Merge branch 'x86-urgent-for-linus' of git://git...
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=1723f40c600000
console output: https://syzkaller.appspot.com/x/log.txt?x=1323f40c600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7937b718ddac333b
dashboard link: https://syzkaller.appspot.com/bug?extid=7692cea7450c97fa2a0a
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17e888cc600000

Reported-by: syzbot+7692cea7450c97fa2a0a@syzkaller.appspotmail.com
Fixes: 1cf8dfe8a661 ("perf/core: Fix race between close() and fork()")

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