Great story! I've had a lot of debugging nightmares, but thankfully never anything as bad as that.

One thing that looks fishy is this branch:

  if (container_tasks_len == max_container_tasks) {
    printk("cplayground: ERROR: container_tasks list hit capacity! We "
    "may be missing processes from the procfile output.\n");
    break;
  }

Since you said printk can block, why isn't calling it in the rcu critical section a bug? Is it because you immediately break afterwards and don't try to reference the next task?