cgroup fix for v5.6-rc1

Hello, Linus.

I made a mistake while removing cgroup task list lazy init optimization making the root cgroup.procs show entries for the init_tasks. The 0 entries doesn't cause critical failures but does make systemd print out warning messages during boot. Fix it by omitting init_tasks as they should be.

Thanks.

The following changes since commit 39bed42de2e7d74686a2d5a45638d6a5d7e7d473:

Merge tag 'for-linus-hmm' of git://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma (2020-01-29 19:56:50 -0800)

are available in the Git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-5.6-fixes

for you to fetch changes up to 0cd9d33ace336bc424fc30944aa3defd6786e4fe:

cgroup: init_tasks shouldn't be linked to the root cgroup (2020-01-30 11:37:33 -0500)

----------------------------------------------------------------
Tejun Heo (1):
cgroup: init_tasks shouldn't be linked to the root cgroup

 kernel/cgroup/cgroup.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

index b3744872263e..cf8a36bdf5c8 100644
@@ -5932,11 +5932,14 @@ void cgroup_post_fork(struct task_struct *child)
 
 	spin_lock_irq(&css_set_lock);
 
-	WARN_ON_ONCE(!list_empty(&child->cg_list));
-	cset = task_css_set(current); /* current is @child's parent */
-	get_css_set(cset);
-	cset->nr_tasks++;
-	css_set_move_task(child, NULL, cset, false);
+	/* init tasks are special, only link regular threads */
+	if (likely(child->pid)) {
+		WARN_ON_ONCE(!list_empty(&child->cg_list));
+		cset = task_css_set(current); /* current is @child's parent */
+		get_css_set(cset);
+		cset->nr_tasks++;
+		css_set_move_task(child, NULL, cset, false);
+	}
 
 	/*
 	 * If the cgroup has to be frozen, the new task has too.  Let's set