Patch "cgroup: make sure a parent css isn't offlined before its children" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

cgroup: make sure a parent css isn't offlined before its children

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
cgroup-make-sure-a-parent-css-isn-t-offlined-before-its-children.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Thu, 21 Jan 2016 15:31:11 -0500
Subject: cgroup: make sure a parent css isn't offlined before its children

From: Tejun Heo <[email protected]>

commit aa226ff4a1ce79f229c6b7a4c0a14e17fececd01 upstream.

There are three subsystem callbacks in css shutdown path - css_offline(), css_released() and css_free(). Except for css_released(), cgroup core didn't guarantee the order of invocation. css_offline() or css_free() could be called on a parent css before its children. This behavior is unexpected and led to bugs in cpu and memory controller.

This patch updates offline path so that a parent css is never offlined before its children. Each css keeps online_cnt which reaches zero iff itself and all its children are offline and offline_css() is invoked only after online_cnt reaches zero.

This fixes the memory controller bug and allows the fix for cpu controller.

---
 include/linux/cgroup-defs.h |    6 ++++++
 kernel/cgroup.c             |   22 +++++++++++++++++-----
 2 files changed, 23 insertions(+), 5 deletions(-)

@@ -133,6 +133,12 @@ struct cgroup_subsys_state {
 	 */
 	u64 serial_nr;
 
+	/*
+	 * Incremented by online self and children.  Used to guarantee that
+	 * parents are not offlined before their children.
+	 */
+	atomic_t online_cnt;
+
 	/* percpu_ref killing and RCU release */
 	struct rcu_head rcu_head;
 	struct work_struct destroy_work;

@@ -4784,6 +4784,7 @@ static void init_and_link_css(struct cgr
 	INIT_LIST_HEAD(&css->sibling);
 	INIT_LIST_HEAD(&css->children);
 	css->serial_nr = css_serial_nr_next++;
+	atomic_set(&css->online_cnt, 0);
 
 	if (cgroup_parent(cgrp)) {
 		css->parent = cgroup_css(cgroup_parent(cgrp), ss);

@@ -4806,6 +4807,10 @@ static int online_css(struct cgroup_subs
 	if (!ret) {
 		css->flags |= CSS_ONLINE;
 		rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
+
+		atomic_inc(&css->online_cnt);
+		if (css->parent)
+			atomic_inc(&css->parent->online_cnt);
 	}
 	return ret;
 }

@@ -5037,10 +5042,15 @@ static void css_killed_work_fn(struct wo
 		container_of(work, struct cgroup_subsys_state, destroy_work);
 
 	mutex_lock(&cgroup_mutex);
-	offline_css(css);
-	mutex_unlock(&cgroup_mutex);
 
-	css_put(css);
+	do {
+		offline_css(css);
+		css_put(css);
+		/* @css can't go away while we're holding cgroup_mutex */
+		css = css->parent;
+	} while (css && atomic_dec_and_test(&css->online_cnt));
+
+	mutex_unlock(&cgroup_mutex);
 }
 
 /* css kill confirmation processing requires process context, bounce */

@@ -5049,8 +5059,10 @@ static void css_killed_ref_fn(struct per
 	struct cgroup_subsys_state *css =
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
-	INIT_WORK(&css->destroy_work, css_killed_work_fn);
-	queue_work(cgroup_destroy_wq, &css->destroy_work);
+	if (atomic_dec_and_test(&css->online_cnt)) {
+		INIT_WORK(&css->destroy_work, css_killed_work_fn);
+		queue_work(cgroup_destroy_wq, &css->destroy_work);
+	}
 }
 
 /**

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/writeback-keep-superblock-pinned-during-cgroup-writeback-association-switches.patch queue-4.4/workqueue-handle-numa_no_node-for-unbound-pool_workqueue-lookup.patch queue-4.4/cgroup-make-sure-a-parent-css-isn-t-offlined-before-its-children.patch queue-4.4/cpuset-make-mm-migration-asynchronous.patch queue-4.4/libata-fix-sff-host-state-machine-locking-while-polling.patch queue-4.4/revert-workqueue-make-sure-delayed-work-run-in-local-cpu.patch