sched: fix copy_namespace() <-> sched_fork() dependency in do_fork

Sukadev Bhattiprolu reported a kernel crash with control groups.
There are couple of problems discovered by Suka's test:

- The test requires the cgroup filesystem to be mounted with
atleast the cpu and ns options (i.e both namespace and cpu
controllers are active in the same hierarchy).

# mkdir /dev/cpuctl
# mount -t cgroup -ocpu,ns none cpuctl
(or simply)
# mount -t cgroup none cpuctl -> Will activate all controllers
in same hierarchy.

- The test invokes clone() with CLONE_NEWNS set. This causes a a new child
to be created, also a new group (do_fork->copy_namespaces->ns_cgroup_clone->
cgroup_clone) and the child is attached to the new group (cgroup_clone->
attach_task->sched_move_task). At this point in time, the child's scheduler
related fields are uninitialized (including its on_rq field, which it has
inherited from parent). As a result sched_move_task thinks its on
runqueue, when it isn't.

As a solution to this problem, I moved sched_fork() call, which
initializes scheduler related fields on a new task, before
copy_namespaces(). I am not sure though whether moving up will
cause other side-effects. Do you see any issue?

- The second problem exposed by this test is that task_new_fair()
assumes that parent and child will be part of the same group (which
needn't be as this test shows). As a result, cfs_rq->curr can be NULL
for the child.

The solution is to test for curr pointer being NULL in
task_new_fair().

With the patch below, I could run ns_exec() fine w/o a crash.

Reported-by: Sukadev Bhattiprolu <[e-mail]>
Signed-off-by: Srivatsa Vaddagiri <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/fork.c

···
 	p->blocked_on = NULL; /* not blocked yet */
 #endif
 
+	/* Perform scheduler related setup. Assign this task to a CPU. */
+	sched_fork(p, clone_flags);
+
 	if ((retval = security_task_alloc(p)))
 		goto bad_fork_cleanup_policy;
 	if ((retval = audit_alloc(p)))
···
 	INIT_LIST_HEAD(&p->thread_group);
 	INIT_LIST_HEAD(&p->ptrace_children);
 	INIT_LIST_HEAD(&p->ptrace_list);
-
-	/* Perform scheduler related setup. Assign this task to a CPU. */
-	sched_fork(p, clone_flags);
 
 	/* Now that the task is set up, run cgroup callbacks if
 	 * necessary. We need to run them before the task is visible

kernel/sched_fair.c

···
 	update_curr(cfs_rq);
 	place_entity(cfs_rq, se, 1);
 
+	/* 'curr' will be NULL if the child belongs to a different group */
 	if (sysctl_sched_child_runs_first && this_cpu == task_cpu(p) &&
-			curr->vruntime < se->vruntime) {
+			curr && curr->vruntime < se->vruntime) {
 		/*
 		 * Upon rescheduling, sched_class::put_prev_task() will place
 		 * 'current' within the tree based on its new key value.