cgroup: implement cgroup_has_tasks() and unexport cgroup_task_count()

cgroup_task_count() read-locks css_set_lock and walks all tasks to
count them and then returns the result.  The only thing all the users
want is determining whether the cgroup is empty or not.  This patch
implements cgroup_has_tasks() which tests whether cgroup->cset_links
is empty, replaces all cgroup_task_count() usages and unexports it.

Note that the test isn't synchronized.  This is the same as before.
The test has always been racy.

This will help planned css_set locking update.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

include/linux/cgroup.h

@@ -457,6 +457,12 @@ static inline bool cgroup_sane_behavior(const struct cgroup *cgrp)
 	return cgrp->root->flags & CGRP_ROOT_SANE_BEHAVIOR;
 }
 
+/* no synchronization, the result can only be used as a hint */
+static inline bool cgroup_has_tasks(struct cgroup *cgrp)
+{
+	return !list_empty(&cgrp->cset_links);
+}
+
 /* returns ino associated with a cgroup, 0 indicates unmounted root */
 static inline ino_t cgroup_ino(struct cgroup *cgrp)
 {
@@ -516,8 +522,6 @@ int cgroup_rm_cftypes(struct cftype *cfts);
 
 bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor);
 
-int cgroup_task_count(const struct cgroup *cgrp);
-
 /*
  * Control Group taskset, used to pass around set of tasks to cgroup_subsys
  * methods.

kernel/cgroup.c

@@ -2391,7 +2391,7 @@ EXPORT_SYMBOL_GPL(cgroup_add_cftypes);
  *
  * Return the number of tasks in the cgroup.
  */
-int cgroup_task_count(const struct cgroup *cgrp)
+static int cgroup_task_count(const struct cgroup *cgrp)
 {
 	int count = 0;
 	struct cgrp_cset_link *link;

kernel/cpuset.c

@@ -467,7 +467,7 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
 	 * be changed to have empty cpus_allowed or mems_allowed.
 	 */
 	ret = -ENOSPC;
-	if ((cgroup_task_count(cur->css.cgroup) || cur->attach_in_progress)) {
+	if ((cgroup_has_tasks(cur->css.cgroup) || cur->attach_in_progress)) {
 		if (!cpumask_empty(cur->cpus_allowed) &&
 		    cpumask_empty(trial->cpus_allowed))
 			goto out;

mm/memcontrol.c

@@ -4958,7 +4958,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
 	struct cgroup *cgrp = memcg->css.cgroup;
 
 	/* returns EBUSY if there is a task or if we come here twice. */
-	if (cgroup_task_count(cgrp) || !list_empty(&cgrp->children))
+	if (cgroup_has_tasks(cgrp) || !list_empty(&cgrp->children))
 		return -EBUSY;
 
 	/* we call try-to-free pages for make this cgroup empty */
@@ -5140,7 +5140,7 @@ static int __memcg_activate_kmem(struct mem_cgroup *memcg,
 	 * of course permitted.
 	 */
 	mutex_lock(&memcg_create_mutex);
-	if (cgroup_task_count(memcg->css.cgroup) || memcg_has_children(memcg))
+	if (cgroup_has_tasks(memcg->css.cgroup) || memcg_has_children(memcg))
 		err = -EBUSY;
 	mutex_unlock(&memcg_create_mutex);
 	if (err)