cgroup: change cgroup_from_id() to css_from_id()

Now we want cgroup core to always provide the css to use to the
subsystems, so change this API to css_from_id().

Uninline css_from_id(), because it's getting bigger and cgroup_css()
has been unexported.

While at it, remove the #ifdef, and shuffle the order of the args.

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

1 files changed, 22 insertions, 0 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b69b572131e5..ff7d642a070a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5717,6 +5717,28 @@ struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
         return css ? css : ERR_PTR(-ENOENT);
 }
 
+/**
+ * css_from_id - lookup css by id
+ * @id: the cgroup id
+ * @ss: cgroup subsys to be looked into
+ *
+ * Returns the css if there's valid one with @id, otherwise returns NULL.
+ * Should be called under rcu_read_lock().
+ */
+struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
+{
+        struct cgroup *cgrp;
+
+        rcu_lockdep_assert(rcu_read_lock_held() ||
+                           lockdep_is_held(&cgroup_mutex),
+                           "css_from_id() needs proper protection");
+
+        cgrp = idr_find(&ss->root->cgroup_idr, id);
+        if (cgrp)
+                return cgroup_css(cgrp, ss->subsys_id);
+        return NULL;
+}
+
 #ifdef CONFIG_CGROUP_DEBUG
 static struct cgroup_subsys_state *
 debug_css_alloc(struct cgroup_subsys_state *parent_css)