cgroup: make css_for_each_descendant() and friends include the origin css in the iteration

Previously, all css descendant iterators didn't include the origin
(root of subtree) css in the iteration.  The reasons were maintaining
consistency with css_for_each_child() and that at the time of
introduction more use cases needed skipping the origin anyway;
however, given that css_is_descendant() considers self to be a
descendant, omitting the origin css has become more confusing and
looking at the accumulated use cases rather clearly indicates that
including origin would result in simpler code overall.

While this is a change which can easily lead to subtle bugs, cgroup
API including the iterators has recently gone through major
restructuring and no out-of-tree changes will be applicable without
adjustments making this a relatively acceptable opportunity for this
type of change.

The conversions are mostly straight-forward.  If the iteration block
had explicit origin handling before or after, it's moved inside the
iteration.  If not, if (pos == origin) continue; is added.  Some
conversions add extra reference get/put around origin handling by
consolidating origin handling and the rest.  While the extra ref
operations aren't strictly necessary, this shouldn't cause any
noticeable difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Aristeu Rozanski <aris@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>

1 files changed, 11 insertions, 18 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index c02a288a4e3..52f0498db94 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2868,17 +2868,6 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
 
 	mutex_unlock(&cgroup_mutex);
 
-	/* @root always needs to be updated */
-	inode = root->dentry->d_inode;
-	mutex_lock(&inode->i_mutex);
-	mutex_lock(&cgroup_mutex);
-	ret = cgroup_addrm_files(root, cfts, is_add);
-	mutex_unlock(&cgroup_mutex);
-	mutex_unlock(&inode->i_mutex);
-
-	if (ret)
-		goto out_deact;
-
 	/* add/rm files for all cgroups created before */
 	rcu_read_lock();
 	css_for_each_descendant_pre(css, cgroup_css(root, ss->subsys_id)) {
@@ -2907,7 +2896,6 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
 	}
 	rcu_read_unlock();
 	dput(prev);
-out_deact:
 	deactivate_super(sb);
 	return ret;
 }
@@ -3099,7 +3087,8 @@ EXPORT_SYMBOL_GPL(css_next_child);
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_pre().  Find the next descendant
- * to visit for pre-order traversal of @root's descendants.
+ * to visit for pre-order traversal of @root's descendants.  @root is
+ * included in the iteration and the first node to be visited.
  *
  * While this function requires RCU read locking, it doesn't require the
  * whole traversal to be contained in a single RCU critical section.  This
@@ -3114,9 +3103,9 @@ css_next_descendant_pre(struct cgroup_subsys_state *pos,
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
 
-	/* if first iteration, pretend we just visited @root */
+	/* if first iteration, visit @root */
 	if (!pos)
-		pos = root;
+		return root;
 
 	/* visit the first child if exists */
 	next = css_next_child(NULL, pos);
@@ -3186,7 +3175,8 @@ css_leftmost_descendant(struct cgroup_subsys_state *pos)
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_post().  Find the next descendant
- * to visit for post-order traversal of @root's descendants.
+ * to visit for post-order traversal of @root's descendants.  @root is
+ * included in the iteration and the last node to be visited.
  *
  * While this function requires RCU read locking, it doesn't require the
  * whole traversal to be contained in a single RCU critical section.  This
@@ -3207,14 +3197,17 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
 		return next != root ? next : NULL;
 	}
 
+	/* if we visited @root, we're done */
+	if (pos == root)
+		return NULL;
+
 	/* if there's an unvisited sibling, visit its leftmost descendant */
 	next = css_next_child(pos, css_parent(pos));
 	if (next)
 		return css_leftmost_descendant(next);
 
 	/* no sibling left, visit parent */
-	next = css_parent(pos);
-	return next != root ? next : NULL;
+	return css_parent(pos);
 }
 EXPORT_SYMBOL_GPL(css_next_descendant_post);