[PATCH] sys_getppid oopses on debug kernel

}

/*

 * Accessing ->group_leader->real_parent is not SMP-safe, it could

 * change from under us. However, rather than getting any lock

 * we can use an optimistic algorithm: get the parent

 * pid, and go back and check that the parent is still

 * the same. If it has changed (which is extremely unlikely

 * indeed), we just try again..

 *

 * NOTE! This depends on the fact that even if we _do_

 * get an old value of "parent", we can happily dereference

 * the pointer (it was and remains a dereferencable kernel pointer

 * no matter what): we just can't necessarily trust the result

 * until we know that the parent pointer is valid.

 *

 * NOTE2: ->group_leader never changes from under us.

 * Accessing ->real_parent is not SMP-safe, it could

 * change from under us. However, we can use a stale

 * value of ->real_parent under rcu_read_lock(), see

 * release_task()->call_rcu(delayed_put_task_struct).

 */

asmlinkage long sys_getppid(void)

{

	int pid;

	struct task_struct *me = current;

	struct task_struct *parent;

	parent = me->group_leader->real_parent;

	for (;;) {

		pid = parent->tgid;

#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)

{

		struct task_struct *old = parent;

	rcu_read_lock();

	pid = rcu_dereference(current->real_parent)->tgid;

	rcu_read_unlock();

		/*

		 * Make sure we read the pid before re-reading the

		 * parent pointer:

		 */

		smp_rmb();

		parent = me->group_leader->real_parent;

		if (old != parent)

			continue;

}

#endif

		break;

	}

	return pid;

}