This patch adds support for subsys.use_id in module-loadable subsystems.

From: Ben Blum <bblum@andrew.cmu.edu>

Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>
---
 kernel/cgroup.c |   23 +++++++++++++++++------
 1 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index cc2e1f6..5af59eb 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -240,7 +240,8 @@ struct cg_cgroup_link {
 static struct css_set init_css_set;
 static struct cg_cgroup_link init_css_set_link;
 
-static int cgroup_subsys_init_idr(struct cgroup_subsys *ss);
+static int cgroup_init_idr(struct cgroup_subsys *ss,
+			   struct cgroup_subsys_state *css);
 
 /* css_set_lock protects the list of css_set objects, and the
  * chain of tasks off each css_set.  Nests outside task->alloc_lock
@@ -3390,6 +3391,16 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
 		mutex_unlock(&cgroup_mutex);
 		return PTR_ERR(css);
 	}
+	/* call init_idr here because it has a failure case */
+	if (ss->use_id) {
+		int ret = cgroup_init_idr(ss, css);
+		if (ret) {
+			ss->destroy(ss, dummytop);
+			subsys[i] = NULL;
+			mutex_unlock(&cgroup_mutex);
+			return ret;
+		}
+	}
 
 	list_add(&ss->sibling, &rootnode.subsys_list);
 	ss->root = &rootnode;
@@ -3484,7 +3495,8 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
 	/*
 	 * remove subsystem's css from the dummytop and free it - need to free
 	 * before marking as null because ss->destroy needs the cgrp->subsys
-	 * pointer to find their state.
+	 * pointer to find their state. note that this also takes care of
+	 * freeing the css_id.
 	 */
 	ss->destroy(ss, dummytop);
 	dummytop->subsys[ss->subsys_id] = NULL;
@@ -3563,7 +3575,7 @@ int __init cgroup_init(void)
 		if (!ss->early_init)
 			cgroup_init_subsys(ss);
 		if (ss->use_id)
-			cgroup_subsys_init_idr(ss);
+			cgroup_init_idr(ss, init_css_set.subsys[ss->subsys_id]);
 	}
 
 	/* Add init_css_set to the hash table */
@@ -4231,15 +4243,14 @@ err_out:
 
 }
 
-static int __init cgroup_subsys_init_idr(struct cgroup_subsys *ss)
+static int __init_or_module cgroup_init_idr(struct cgroup_subsys *ss,
+					    struct cgroup_subsys_state *rootcss)
 {
 	struct css_id *newid;
-	struct cgroup_subsys_state *rootcss;
 
 	spin_lock_init(&ss->id_lock);
 	idr_init(&ss->idr);
 
-	rootcss = init_css_set.subsys[ss->subsys_id];
 	newid = get_new_cssid(ss, 0);
 	if (IS_ERR(newid))
 		return PTR_ERR(newid);