cgroup: convert cgroup_cft_commit() to use cgroup_for_each_descendant_pre()

We used root->allcg_list to iterate cgroup hierarchy because at that time
cgroup_for_each_descendant_pre() hasn't been invented.

tj: In cgroup_cfts_commit(), s/@serial_nr/@update_upto/, move the
    assignment right above releasing cgroup_mutex and explain what's
    going on there.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -206,9 +206,6 @@ struct cgroup {
 	 */
 	struct list_head cset_links;
 
-	struct list_head allcg_node;	/* cgroupfs_root->allcg_list */
-	struct list_head cft_q_node;	/* used during cftype add/rm */
-
 	/*
 	 * Linked list running through all cgroups that can
 	 * potentially be reaped by the release agent. Protected by
@@ -313,9 +310,6 @@ struct cgroupfs_root {
 	/* A list running through the active hierarchies */
 	struct list_head root_list;
 
-	/* All cgroups on this root, cgroup_mutex protected */
-	struct list_head allcg_list;
-
 	/* Hierarchy-specific flags */
 	unsigned long flags;
 

kernel/cgroup.c

@@ -1399,7 +1399,6 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
 	INIT_LIST_HEAD(&cgrp->children);
 	INIT_LIST_HEAD(&cgrp->files);
 	INIT_LIST_HEAD(&cgrp->cset_links);
-	INIT_LIST_HEAD(&cgrp->allcg_node);
 	INIT_LIST_HEAD(&cgrp->release_list);
 	INIT_LIST_HEAD(&cgrp->pidlists);
 	mutex_init(&cgrp->pidlist_mutex);
@@ -1414,12 +1413,10 @@ static void init_cgroup_root(struct cgroupfs_root *root)
 
 	INIT_LIST_HEAD(&root->subsys_list);
 	INIT_LIST_HEAD(&root->root_list);
-	INIT_LIST_HEAD(&root->allcg_list);
 	root->number_of_cgroups = 1;
 	cgrp->root = root;
 	cgrp->name = &root_cgroup_name;
 	init_cgroup_housekeeping(cgrp);
-	list_add_tail(&cgrp->allcg_node, &root->allcg_list);
 }
 
 static int cgroup_init_root_id(struct cgroupfs_root *root)
@@ -2785,65 +2782,78 @@ static int cgroup_addrm_files(struct cgroup *cgrp, struct cgroup_subsys *subsys,
 	return ret;
 }
 
-static DEFINE_MUTEX(cgroup_cft_mutex);
-
 static void cgroup_cfts_prepare(void)
-	__acquires(&cgroup_cft_mutex) __acquires(&cgroup_mutex)
+	__acquires(&cgroup_mutex)
 {
 	/*
 	 * Thanks to the entanglement with vfs inode locking, we can't walk
 	 * the existing cgroups under cgroup_mutex and create files.
-	 * Instead, we increment reference on all cgroups and build list of
-	 * them using @cgrp->cft_q_node.  Grab cgroup_cft_mutex to ensure
-	 * exclusive access to the field.
+	 * Instead, we use cgroup_for_each_descendant_pre() and drop RCU
+	 * read lock before calling cgroup_addrm_files().
 	 */
-	mutex_lock(&cgroup_cft_mutex);
 	mutex_lock(&cgroup_mutex);
 }
 
 static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 			       struct cftype *cfts, bool is_add)
-	__releases(&cgroup_mutex) __releases(&cgroup_cft_mutex)
+	__releases(&cgroup_mutex)
 {
 	LIST_HEAD(pending);
-	struct cgroup *cgrp, *n;
+	struct cgroup *cgrp, *root = &ss->root->top_cgroup;
 	struct super_block *sb = ss->root->sb;
+	struct dentry *prev = NULL;
+	struct inode *inode;
+	u64 update_upto;
 
 	/* %NULL @cfts indicates abort and don't bother if @ss isn't attached */
-	if (cfts && ss->root != &rootnode &&
-	    atomic_inc_not_zero(sb->s_active)) {
-		list_for_each_entry(cgrp, &ss->root->allcg_list, allcg_node) {
-			dget(cgrp->dentry);
-			list_add_tail(&cgrp->cft_q_node, &pending);
-		}
-	} else {
-		sb = NULL;
+	if (!cfts || ss->root == &rootnode ||
+	    !atomic_inc_not_zero(&sb->s_active)) {
+		mutex_unlock(&cgroup_mutex);
+		return;
 	}
 
-	mutex_unlock(&cgroup_mutex);
-
 	/*
-	 * All new cgroups will see @cfts update on @ss->cftsets.  Add/rm
-	 * files for all cgroups which were created before.
+	 * All cgroups which are created after we drop cgroup_mutex will
+	 * have the updated set of files, so we only need to update the
+	 * cgroups created before the current @cgroup_serial_nr_cursor.
 	 */
-	list_for_each_entry_safe(cgrp, n, &pending, cft_q_node) {
-		struct inode *inode = cgrp->dentry->d_inode;
+	update_upto = atomic64_read(&cgroup_serial_nr_cursor);
+
+	mutex_unlock(&cgroup_mutex);
+
+	/* @root always needs to be updated */
+	inode = root->dentry->d_inode;
+	mutex_lock(&inode->i_mutex);
+	mutex_lock(&cgroup_mutex);
+	cgroup_addrm_files(root, ss, cfts, is_add);
+	mutex_unlock(&cgroup_mutex);
+	mutex_unlock(&inode->i_mutex);
+
+	/* add/rm files for all cgroups created before */
+	rcu_read_lock();
+	cgroup_for_each_descendant_pre(cgrp, root) {
+		if (cgroup_is_dead(cgrp))
+			continue;
+
+		inode = cgrp->dentry->d_inode;
+		dget(cgrp->dentry);
+		rcu_read_unlock();
+
+		dput(prev);
+		prev = cgrp->dentry;
 
 		mutex_lock(&inode->i_mutex);
 		mutex_lock(&cgroup_mutex);
-		if (!cgroup_is_dead(cgrp))
+		if (cgrp->serial_nr <= update_upto && !cgroup_is_dead(cgrp))
 			cgroup_addrm_files(cgrp, ss, cfts, is_add);
 		mutex_unlock(&cgroup_mutex);
 		mutex_unlock(&inode->i_mutex);
 
-		list_del_init(&cgrp->cft_q_node);
-		dput(cgrp->dentry);
+		rcu_read_lock();
 	}
-
-	if (sb)
-		deactivate_super(sb);
-
-	mutex_unlock(&cgroup_cft_mutex);
+	rcu_read_unlock();
+	dput(prev);
+	deactivate_super(sb);
 }
 
 /**
@@ -4320,7 +4330,6 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 	cgrp->serial_nr = atomic64_inc_return(&cgroup_serial_nr_cursor);
 
 	/* allocation complete, commit to creation */
-	list_add_tail(&cgrp->allcg_node, &root->allcg_list);
 	list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
 	root->number_of_cgroups++;
 
@@ -4559,7 +4568,6 @@ static void cgroup_offline_fn(struct work_struct *work)
 
 	/* delete this cgroup from parent->children */
 	list_del_rcu(&cgrp->sibling);
-	list_del_init(&cgrp->allcg_node);
 
 	dput(d);