[PATCH 4.4 024/163] sched/cgroup: Fix/cleanup cgroup teardown/init

From: Greg Kroah-Hartman
Date: Mon May 02 2016 - 21:49:51 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

commit 2f5177f0fd7e531b26d54633be62d1d4cb94621c upstream.

The CPU controller hasn't kept up with the various changes in the whole
cgroup initialization / destruction sequence, and commit:

2e91fa7f6d45 ("cgroup: keep zombies associated with their original cgroups")

caused it to explode.

The reason for this is that zombies do not inhibit css_offline() from
being called, but do stall css_released(). Now we tear down the cfs_rq
structures on css_offline() but zombies can run after that, leading to
use-after-free issues.

The solution is to move the tear-down to css_released(), which
guarantees nobody (including no zombies) is still using our cgroup.

Furthermore, a few simple cleanups are possible too. There doesn't
appear to be any point to us using css_online() (anymore?) so fold that
in css_alloc().

And since cgroup code guarantees an RCU grace period between
css_released() and css_free() we can forgo using call_rcu() and free the
stuff immediately.

Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Kazuki Yamaguchi <k@xxxxxx>
Reported-by: Niklas Cassel <niklas.cassel@xxxxxxxx>
Tested-by: Niklas Cassel <niklas.cassel@xxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Fixes: 2e91fa7f6d45 ("cgroup: keep zombies associated with their original cgroups")
Link: http://lkml.kernel.org/r/20160316152245.GY6344@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

kernel/sched/core.c | 35 ++++++++++++++---------------------
1 file changed, 14 insertions(+), 21 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7693,7 +7693,7 @@ void set_curr_task(int cpu, struct task_
/* task_group_lock serializes the addition/removal of task groups */
static DEFINE_SPINLOCK(task_group_lock);

-static void free_sched_group(struct task_group *tg)
+static void sched_free_group(struct task_group *tg)
@@ -7719,7 +7719,7 @@ struct task_group *sched_create_group(st
return tg;

- free_sched_group(tg);
+ sched_free_group(tg);
return ERR_PTR(-ENOMEM);

@@ -7739,17 +7739,16 @@ void sched_online_group(struct task_grou

/* rcu callback to free various structures associated with a task group */
-static void free_sched_group_rcu(struct rcu_head *rhp)
+static void sched_free_group_rcu(struct rcu_head *rhp)
/* now it should be safe to free those cfs_rqs */
- free_sched_group(container_of(rhp, struct task_group, rcu));
+ sched_free_group(container_of(rhp, struct task_group, rcu));

-/* Destroy runqueue etc associated with a task group */
void sched_destroy_group(struct task_group *tg)
/* wait for possible concurrent references to cfs_rqs complete */
- call_rcu(&tg->rcu, free_sched_group_rcu);
+ call_rcu(&tg->rcu, sched_free_group_rcu);

void sched_offline_group(struct task_group *tg)
@@ -8210,31 +8209,26 @@ cpu_cgroup_css_alloc(struct cgroup_subsy
if (IS_ERR(tg))
return ERR_PTR(-ENOMEM);

+ sched_online_group(tg, parent);
return &tg->css;

-static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
+static void cpu_cgroup_css_released(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;
+ sched_offline_group(tg);

static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
struct task_group *tg = css_tg(css);

- sched_destroy_group(tg);
-static void cpu_cgroup_css_offline(struct cgroup_subsys_state *css)
- struct task_group *tg = css_tg(css);
- sched_offline_group(tg);
+ /*
+ * Relies on the RCU grace period between css_released() and this.
+ */
+ sched_free_group(tg);

static void cpu_cgroup_fork(struct task_struct *task, void *private)
@@ -8594,9 +8588,8 @@ static struct cftype cpu_files[] = {

struct cgroup_subsys cpu_cgrp_subsys = {
.css_alloc = cpu_cgroup_css_alloc,
+ .css_released = cpu_cgroup_css_released,
.css_free = cpu_cgroup_css_free,
- .css_online = cpu_cgroup_css_online,
- .css_offline = cpu_cgroup_css_offline,
.fork = cpu_cgroup_fork,
.can_attach = cpu_cgroup_can_attach,
.attach = cpu_cgroup_attach,