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>

1 files changed, 44 insertions, 36 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e6571ca822a0..0ed7d8db6508 100644
--- a/kernel/cgroup.c
+++ b/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);