[PATCH RESEND/add comment] sched/cgroup: move sched_online_group() back into css_online()

[Konstantin Khlebnikov]

Commit 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init") moved
sched_online_group() from css_online() to css_alloc(). It exposes half-baked
task group into global lists before initializing generic cgroup stuff.

LTP testcase (third in cgroup_regression_test) written for testing
similar race in kernels 2.6.26-2.6.28 easily triggers this oops:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
IP: kernfs_path_from_node_locked+0x260/0x320
CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
Call Trace:
? kernfs_path_from_node+0x4f/0x60
kernfs_path_from_node+0x3e/0x60
print_rt_rq+0x44/0x2b0
print_rt_stats+0x7a/0xd0
print_cpu+0x2fc/0xe80
? __might_sleep+0x4a/0x80
sched_debug_show+0x17/0x30
seq_read+0xf2/0x3b0
proc_reg_read+0x42/0x70
__vfs_read+0x28/0x130
? security_file_permission+0x9b/0xc0
? rw_verify_area+0x4e/0xb0
vfs_read+0xa5/0x170
SyS_read+0x46/0xa0
entry_SYSCALL_64_fastpath+0x1e/0xad

Here task group already linked into global RCU-protected list task_groups
but pointer css->cgroup is still NULL.

This patch reverts this chunk and moves online back to css_online().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
Cc: <stable@xxxxxxxxxxxxxxx> # 4.6+
---
 kernel/sched/core.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c56fb57f2991..5bfd9cc2aa29 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8398,11 +8398,20 @@ cpu_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 	if (IS_ERR(tg))
 		return ERR_PTR(-ENOMEM);
 
-	sched_online_group(tg, parent);
-
 	return &tg->css;
 }
 
+/* Expose task group only after completing cgroup initialization */
+static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
+{
+	struct task_group *tg = css_tg(css);
+	struct task_group *parent = css_tg(css->parent);
+
+	if (parent)
+		sched_online_group(tg, parent);
+	return 0;
+}
+
 static void cpu_cgroup_css_released(struct cgroup_subsys_state *css)
 {
 	struct task_group *tg = css_tg(css);
@@ -8805,6 +8814,7 @@ static struct cftype cpu_files[] = {
 
 struct cgroup_subsys cpu_cgrp_subsys = {
 	.css_alloc	= cpu_cgroup_css_alloc,
+	.css_online	= cpu_cgroup_css_online,
 	.css_released	= cpu_cgroup_css_released,
 	.css_free	= cpu_cgroup_css_free,
 	.fork		= cpu_cgroup_fork,