[PATCH 3/9] use hierarchy mutex in memcg

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

This was RFC from Paul Menage, just for base of my series.
==
From:	menage@google.com

This patch updates the memory controller to use its hierarchy_mutex
rather than calling cgroup_lock() to protected against
cgroup_mkdir()/cgroup_rmdir() from occurring in its hierarchy.

Signed-off-by: Paul Menage <menage@google.com>

---
Index: mmotm-2.6.28-Dec12/mm/memcontrol.c
===================================================================
--- mmotm-2.6.28-Dec12.orig/mm/memcontrol.c
+++ mmotm-2.6.28-Dec12/mm/memcontrol.c
@@ -154,7 +154,7 @@ struct mem_cgroup {
 
 	/*
 	 * While reclaiming in a hiearchy, we cache the last child we
-	 * reclaimed from. Protected by cgroup_lock()
+	 * reclaimed from. Protected by hierarchy_mutex
 	 */
 	struct mem_cgroup *last_scanned_child;
 	/*
@@ -554,7 +554,7 @@ unsigned long mem_cgroup_isolate_pages(u
 
 /*
  * This routine finds the DFS walk successor. This routine should be
- * called with cgroup_mutex held
+ * called with hierarchy_mutex held
  */
 static struct mem_cgroup *
 mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
@@ -623,7 +623,7 @@ mem_cgroup_get_first_node(struct mem_cgr
 	/*
 	 * Scan all children under the mem_cgroup mem
 	 */
-	cgroup_lock();
+	mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
 	if (list_empty(&root_mem->css.cgroup->children)) {
 		ret = root_mem;
 		goto done;
@@ -644,7 +644,7 @@ mem_cgroup_get_first_node(struct mem_cgr
 
 done:
 	root_mem->last_scanned_child = ret;
-	cgroup_unlock();
+	mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
 	return ret;
 }
 
@@ -708,18 +708,16 @@ static int mem_cgroup_hierarchical_recla
 	while (next_mem != root_mem) {
 		if (next_mem->obsolete) {
 			mem_cgroup_put(next_mem);
-			cgroup_lock();
 			next_mem = mem_cgroup_get_first_node(root_mem);
-			cgroup_unlock();
 			continue;
 		}
 		ret = try_to_free_mem_cgroup_pages(next_mem, gfp_mask, noswap,
 						   get_swappiness(next_mem));
 		if (mem_cgroup_check_under_limit(root_mem))
 			return 0;
-		cgroup_lock();
+		mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
 		next_mem = mem_cgroup_get_next_node(next_mem, root_mem);
-		cgroup_unlock();
+		mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
 	}
 	return ret;
 }

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