This bug is introduced between v3.7 and v3.8, by commit:
af4b8a83add95ef40716401395b44a1b579965f4 pidns: Wait in zap_pid_ns_processes until pid_ns->nr_hashed == 1
This bug is introduced between v3.7 and v3.8, by commit:
af4b8a83add95ef 40716401395b44a 1b579965f4 pidns: Wait in zap_pid_ ns_processes until pid_ns->nr_hashed == 1