sched/cgroup: Move sched_online_group() back into css_online() to fix crash

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

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

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
Link: http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cc1e3e0d29b09672d614a40f683c06d3c7f9839c..e01bd807f0afff31e50aacc444ad4b283bf55c21 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6811,11 +6811,20 @@ 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);
@@ -7221,6 +7230,7 @@ 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,