Patch "cgroup: fix invalid controller enable rejections with cgroup namespace" has been added to the 4.7-stable tree

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

cgroup: fix invalid controller enable rejections with cgroup namespace

to the 4.7-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-fix-invalid-controller-enable-rejections-with-cgroup-namespace.patch
and it can be found in the queue-4.7 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: Fri, 23 Sep 2016 16:55:49 -0400
Subject: cgroup: fix invalid controller enable rejections with cgroup namespace

From: Tejun Heo <[email protected]>

commit 9157056da8f8c4a6305f15619e269f164b63a6de upstream.

On the v2 hierarchy, "cgroup.subtree_control" rejects controller enables if the cgroup has processes in it. The enforcement of this logic assumes that the cgroup wouldn't have any css_sets associated with it if there are no tasks in the cgroup, which is no longer true since a79a908fd2b0 ("cgroup: introduce cgroup namespaces").

When a cgroup namespace is created, it pins the css_set of the creating task to use it as the root css_set of the namespace. This extra reference stays as long as the namespace is around and makes "cgroup.subtree_control" think that the namespace root cgroup is not empty even when it is and thus reject controller enables.

Fix it by making cgroup_subtree_control() walk and test emptiness of each css_set instead of testing whether the list_head is empty.

While at it, update the comment of cgroup_task_count() to indicate that the returned value may be higher than the number of tasks, which has always been true due to temporary references and doesn't break anything.

---
 kernel/cgroup.c |   29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

@@ -3452,9 +3452,28 @@ static ssize_t cgroup_subtree_control_wr
 	 * Except for the root, subtree_control must be zero for a cgroup
 	 * with tasks so that child cgroups don't compete against tasks.
 	 */
-	if (enable && cgroup_parent(cgrp) && !list_empty(&cgrp->cset_links)) {
-		ret = -EBUSY;
-		goto out_unlock;
+	if (enable && cgroup_parent(cgrp)) {
+		struct cgrp_cset_link *link;
+
+		/*
+		 * Because namespaces pin csets too, @cgrp->cset_links
+		 * might not be empty even when @cgrp is empty.  Walk and
+		 * verify each cset.
+		 */
+		spin_lock_irq(&css_set_lock);
+
+		ret = 0;
+		list_for_each_entry(link, &cgrp->cset_links, cset_link) {
+			if (css_set_populated(link->cset)) {
+				ret = -EBUSY;
+				break;
+			}
+		}
+
+		spin_unlock_irq(&css_set_lock);
+
+		if (ret)
+			goto out_unlock;
 	}
 
 	/* save and update control masks and prepare csses */

@@ -3905,7 +3924,9 @@ void cgroup_file_notify(struct cgroup_fi
  * cgroup_task_count - count the number of tasks in a cgroup.
  * @cgrp: the cgroup in question
  *
- * Return the number of tasks in the cgroup.
+ * Return the number of tasks in the cgroup.  The returned number can be
+ * higher than the actual number of tasks due to css_set references from
+ * namespace roots and temporary usages.
  */
 static int cgroup_task_count(const struct cgroup *cgrp)
 {

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

queue-4.7/cpuset-handle-race-between-cpu-hotplug-and-cpuset_hotplug_work.patch queue-4.7/cgroup-fix-invalid-controller-enable-rejections-with-cgroup-namespace.patch