cgroup: replace task_cgroup_path_from_hierarchy() with task_cgroup_path()

task_cgroup_path_from_hierarchy() was added for the planned new users
and none of the currently planned users wants to know about multiple
hierarchies. This patch drops the multiple hierarchy part and makes
it always return the path in the first non-dummy hierarchy.

As unified hierarchy will always have id 1, this is guaranteed to
return the path for the unified hierarchy if mounted; otherwise, it
will return the path from the hierarchy which happens to occupy the
lowest hierarchy id, which will usually be the first hierarchy mounted
after boot.

Signed-off-by: Tejun Heo <[e-mail]>
Acked-by: Li Zefan <[e-mail]>
Cc: Lennart Poettering <[e-mail]>
Cc: Kay Sievers <[e-mail]>
Cc: Jan Kaluža <[e-mail]>

include/linux/cgroup.h

···
 bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor);
 
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen);
-int task_cgroup_path_from_hierarchy(struct task_struct *task, int hierarchy_id,
-				    char *buf, size_t buflen);
+int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen);
 
 int cgroup_task_count(const struct cgroup *cgrp);
 

kernel/cgroup.c

···
 EXPORT_SYMBOL_GPL(cgroup_path);
 
 /**
- * task_cgroup_path_from_hierarchy - cgroup path of a task on a hierarchy
+ * task_cgroup_path - cgroup path of a task in the first cgroup hierarchy
  * @task: target task
- * @hierarchy_id: the hierarchy to look up @task's cgroup from
  * @buf: the buffer to write the path into
  * @buflen: the length of the buffer
  *
- * Determine @task's cgroup on the hierarchy specified by @hierarchy_id and
- * copy its path into @buf.  This function grabs cgroup_mutex and shouldn't
- * be used inside locks used by cgroup controller callbacks.
+ * Determine @task's cgroup on the first (the one with the lowest non-zero
+ * hierarchy_id) cgroup hierarchy and copy its path into @buf.  This
+ * function grabs cgroup_mutex and shouldn't be used inside locks used by
+ * cgroup controller callbacks.
+ *
+ * Returns 0 on success, fails with -%ENAMETOOLONG if @buflen is too short.
  */
-int task_cgroup_path_from_hierarchy(struct task_struct *task, int hierarchy_id,
-				    char *buf, size_t buflen)
+int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
 {
 	struct cgroupfs_root *root;
-	struct cgroup *cgrp = NULL;
-	int ret = -ENOENT;
+	struct cgroup *cgrp;
+	int hierarchy_id = 1, ret = 0;
+
+	if (buflen < 2)
+		return -ENAMETOOLONG;
 
 	mutex_lock(&cgroup_mutex);
 
-	root = idr_find(&cgroup_hierarchy_idr, hierarchy_id);
+	root = idr_get_next(&cgroup_hierarchy_idr, &hierarchy_id);
+
 	if (root) {
 		cgrp = task_cgroup_from_root(task, root);
 		ret = cgroup_path(cgrp, buf, buflen);
+	} else {
+		/* if no hierarchy exists, everyone is in "/" */
+		memcpy(buf, "/", 2);
 	}
 
 	mutex_unlock(&cgroup_mutex);
-
 	return ret;
 }
-EXPORT_SYMBOL_GPL(task_cgroup_path_from_hierarchy);
+EXPORT_SYMBOL_GPL(task_cgroup_path);
 
 /*
  * Control Group taskset