cgroup: remove cgroup->parent

cgroup->parent is redundant as cgroup->self.parent can also be used to
determine the parent cgroup and we're moving towards using
cgroup_subsys_states as the fundamental structural blocks.  This patch
introduces cgroup_parent() which follows cgroup->self.parent and
removes cgroup->parent.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

2 files changed, 30 insertions, 23 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2549493d518d..fd538f4c2bb6 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -178,7 +178,6 @@ struct cgroup {
         struct list_head sibling;       /* my parent's children */
         struct list_head children;      /* my children */
 
-        struct cgroup *parent;          /* my parent */
         struct kernfs_node *kn;         /* cgroup kernfs entry */
         struct kernfs_node *populated_kn; /* kn for "cgroup.subtree_populated" */
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 929bbbc539e9..8c67a739aea4 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -218,6 +218,15 @@ static void cgroup_idr_remove(struct idr *idr, int id)
         spin_unlock_bh(&cgroup_idr_lock);
 }
 
+static struct cgroup *cgroup_parent(struct cgroup *cgrp)
+{
+        struct cgroup_subsys_state *parent_css = cgrp->self.parent;
+
+        if (parent_css)
+                return container_of(parent_css, struct cgroup, self);
+        return NULL;
+}
+
 /**
  * cgroup_css - obtain a cgroup's css for the specified subsystem
  * @cgrp: the cgroup of interest
@@ -260,9 +269,9 @@ static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
         if (!(cgrp->root->subsys_mask & (1 << ss->id)))
                 return NULL;
 
-        while (cgrp->parent &&
-               !(cgrp->parent->child_subsys_mask & (1 << ss->id)))
-                cgrp = cgrp->parent;
+        while (cgroup_parent(cgrp) &&
+               !(cgroup_parent(cgrp)->child_subsys_mask & (1 << ss->id)))
+                cgrp = cgroup_parent(cgrp);
 
         return cgroup_css(cgrp, ss);
 }
@@ -307,7 +316,7 @@ bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor)
         while (cgrp) {
                 if (cgrp == ancestor)
                         return true;
-                cgrp = cgrp->parent;
+                cgrp = cgroup_parent(cgrp);
         }
         return false;
 }
@@ -454,7 +463,7 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 
                 if (cgrp->populated_kn)
                         kernfs_notify(cgrp->populated_kn);
-                cgrp = cgrp->parent;
+                cgrp = cgroup_parent(cgrp);
         } while (cgrp);
 }
 
@@ -2018,7 +2027,7 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
          * Except for the root, child_subsys_mask must be zero for a cgroup
          * with tasks so that child cgroups don't compete against tasks.
          */
-        if (dst_cgrp && cgroup_on_dfl(dst_cgrp) && dst_cgrp->parent &&
+        if (dst_cgrp && cgroup_on_dfl(dst_cgrp) && cgroup_parent(dst_cgrp) &&
             dst_cgrp->child_subsys_mask)
                 return -EBUSY;
 
@@ -2427,7 +2436,7 @@ static int cgroup_controllers_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
 
-        cgroup_print_ss_mask(seq, cgrp->parent->child_subsys_mask);
+        cgroup_print_ss_mask(seq, cgroup_parent(cgrp)->child_subsys_mask);
         return 0;
 }
 
@@ -2610,8 +2619,8 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 
                         /* unavailable or not enabled on the parent? */
                         if (!(cgrp_dfl_root.subsys_mask & (1 << ssid)) ||
-                            (cgrp->parent &&
-                             !(cgrp->parent->child_subsys_mask & (1 << ssid)))) {
+                            (cgroup_parent(cgrp) &&
+                             !(cgroup_parent(cgrp)->child_subsys_mask & (1 << ssid)))) {
                                 ret = -ENOENT;
                                 goto out_unlock;
                         }
@@ -2640,7 +2649,7 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
          * Except for the root, child_subsys_mask must be zero for a cgroup
          * with tasks so that child cgroups don't compete against tasks.
          */
-        if (enable && cgrp->parent && !list_empty(&cgrp->cset_links)) {
+        if (enable && cgroup_parent(cgrp) && !list_empty(&cgrp->cset_links)) {
                 ret = -EBUSY;
                 goto out_unlock;
         }
@@ -2898,9 +2907,9 @@ static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
                         continue;
                 if ((cft->flags & CFTYPE_INSANE) && cgroup_sane_behavior(cgrp))
                         continue;
-                if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgrp->parent)
+                if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgroup_parent(cgrp))
                         continue;
-                if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgrp->parent)
+                if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgroup_parent(cgrp))
                         continue;
 
                 if (is_add) {
@@ -4092,14 +4101,14 @@ static void css_free_work_fn(struct work_struct *work)
                 atomic_dec(&cgrp->root->nr_cgrps);
                 cgroup_pidlist_destroy_all(cgrp);
 
-                if (cgrp->parent) {
+                if (cgroup_parent(cgrp)) {
                         /*
                          * We get a ref to the parent, and put the ref when
                          * this cgroup is being freed, so it's guaranteed
                          * that the parent won't be destroyed before its
                          * children.
                          */
-                        cgroup_put(cgrp->parent);
+                        cgroup_put(cgroup_parent(cgrp));
                         kernfs_put(cgrp->kn);
                         kfree(cgrp);
                 } else {
@@ -4163,8 +4172,8 @@ static void init_and_link_css(struct cgroup_subsys_state *css,
         css->ss = ss;
         css->flags = 0;
 
-        if (cgrp->parent) {
-                css->parent = cgroup_css(cgrp->parent, ss);
+        if (cgroup_parent(cgrp)) {
+                css->parent = cgroup_css(cgroup_parent(cgrp), ss);
                 css_get(css->parent);
         }
 
@@ -4218,7 +4227,7 @@ static void offline_css(struct cgroup_subsys_state *css)
  */
 static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
 {
-        struct cgroup *parent = cgrp->parent;
+        struct cgroup *parent = cgroup_parent(cgrp);
         struct cgroup_subsys_state *css;
         int err;
 
@@ -4251,7 +4260,7 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
                 goto err_clear_dir;
 
         if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
-            parent->parent) {
+            cgroup_parent(parent)) {
                 pr_warn("%s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
                         current->comm, current->pid, ss->name);
                 if (!strcmp(ss->name, "memory"))
@@ -4309,7 +4318,6 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 
         init_cgroup_housekeeping(cgrp);
 
-        cgrp->parent = parent;
         cgrp->self.parent = &parent->self;
         cgrp->root = root;
 
@@ -4336,7 +4344,7 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
         cgrp->serial_nr = cgroup_serial_nr_next++;
 
         /* allocation complete, commit to creation */
-        list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
+        list_add_tail_rcu(&cgrp->sibling, &cgroup_parent(cgrp)->children);
         atomic_inc(&root->nr_cgrps);
         cgroup_get(parent);
 
@@ -4531,8 +4539,8 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
          */
         kernfs_remove(cgrp->kn);
 
-        set_bit(CGRP_RELEASABLE, &cgrp->parent->flags);
-        check_for_release(cgrp->parent);
+        set_bit(CGRP_RELEASABLE, &cgroup_parent(cgrp)->flags);
+        check_for_release(cgroup_parent(cgrp));
 
         /* put the base reference */
         percpu_ref_kill(&cgrp->self.refcnt);