cgroup: convert cgroup_has_live_children() into css_has_online_children()

Now that cgroup liveliness and css onliness are the same state,
convert cgroup_has_live_children() into css_has_online_children() so
that it can be used for actual csses too.  The function now uses
css_for_each_child() for iteration and is published.

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

2 files changed, 22 insertions, 12 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 51a339c99eb6..b76999954beb 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -873,6 +873,8 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
         for ((pos) = css_next_descendant_post(NULL, (css)); (pos);      \
              (pos) = css_next_descendant_post((pos), (css)))
 
+bool css_has_online_children(struct cgroup_subsys_state *css);
+
 /* A css_task_iter should be treated as an opaque object */
 struct css_task_iter {
         struct cgroup_subsys            *ss;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 004004fd0ded..082bb842b11a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -175,7 +175,6 @@ static int need_forkexit_callback __read_mostly;
 static struct cftype cgroup_base_files[];
 
 static void cgroup_put(struct cgroup *cgrp);
-static bool cgroup_has_live_children(struct cgroup *cgrp);
 static int rebind_subsystems(struct cgroup_root *dst_root,
                              unsigned int ss_mask);
 static int cgroup_destroy_locked(struct cgroup *cgrp);
@@ -1769,7 +1768,7 @@ static void cgroup_kill_sb(struct super_block *sb)
          * This prevents new mounts by disabling percpu_ref_tryget_live().
          * cgroup_mount() may wait for @root's release.
          */
-        if (cgroup_has_live_children(&root->cgrp))
+        if (css_has_online_children(&root->cgrp.self))
                 cgroup_put(&root->cgrp);
         else
                 percpu_ref_kill(&root->cgrp.self.refcnt);
@@ -3291,19 +3290,28 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
         return pos->parent;
 }
 
-static bool cgroup_has_live_children(struct cgroup *cgrp)
+/**
+ * css_has_online_children - does a css have online children
+ * @css: the target css
+ *
+ * Returns %true if @css has any online children; otherwise, %false.  This
+ * function can be called from any context but the caller is responsible
+ * for synchronizing against on/offlining as necessary.
+ */
+bool css_has_online_children(struct cgroup_subsys_state *css)
 {
-        struct cgroup *child;
+        struct cgroup_subsys_state *child;
+        bool ret = false;
 
         rcu_read_lock();
-        list_for_each_entry_rcu(child, &cgrp->self.children, self.sibling) {
-                if (!cgroup_is_dead(child)) {
-                        rcu_read_unlock();
-                        return true;
+        css_for_each_child(child, css) {
+                if (css->flags & CSS_ONLINE) {
+                        ret = true;
+                        break;
                 }
         }
         rcu_read_unlock();
-        return false;
+        return ret;
 }
 
 /**
@@ -4535,7 +4543,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
          * ->self.children as dead children linger on it while being
          * drained; otherwise, "rmdir parent/child parent" may fail.
          */
-        if (cgroup_has_live_children(cgrp))
+        if (css_has_online_children(&cgrp->self))
                 return -EBUSY;
 
         /*
@@ -5014,8 +5022,8 @@ void cgroup_exit(struct task_struct *tsk)
 
 static void check_for_release(struct cgroup *cgrp)
 {
-        if (cgroup_is_releasable(cgrp) &&
-            list_empty(&cgrp->cset_links) && !cgroup_has_live_children(cgrp)) {
+        if (cgroup_is_releasable(cgrp) && list_empty(&cgrp->cset_links) &&
+            !css_has_online_children(&cgrp->self)) {
                 /*
                  * Control Group is currently removeable. If it's not
                  * already queued for a userspace notification, queue