My main purpose of this patch is for memory controller..

This patch adds a handler "pre_destroy" to cgroup_subsys.
It is called before cgroup_rmdir() checks all subsys's refcnt.

I think this is useful for subsyses which have some extra refs
even if there are no tasks in cgroup. By adding pre_destroy(),
the kernel keeps the rule "destroy() against subsystem is called only
when refcnt=0." and allows css's ref to be used by other objects
than tasks.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

 include/linux/cgroup.h |    1 +
 kernel/cgroup.c        |    7 +++++++
 2 files changed, 8 insertions(+)

Index: devel-2.6.23-mm1/include/linux/cgroup.h
===================================================================
--- devel-2.6.23-mm1.orig/include/linux/cgroup.h
+++ devel-2.6.23-mm1/include/linux/cgroup.h
@@ -233,6 +233,7 @@ int cgroup_is_descendant(const struct cg
 struct cgroup_subsys {
 	struct cgroup_subsys_state *(*create)(struct cgroup_subsys *ss,
 						  struct cgroup *cont);
+	void (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cont);
 	void (*destroy)(struct cgroup_subsys *ss, struct cgroup *cont);
 	int (*can_attach)(struct cgroup_subsys *ss,
 			  struct cgroup *cont, struct task_struct *tsk);
Index: devel-2.6.23-mm1/kernel/cgroup.c
===================================================================
--- devel-2.6.23-mm1.orig/kernel/cgroup.c
+++ devel-2.6.23-mm1/kernel/cgroup.c
@@ -2158,6 +2158,13 @@ static int cgroup_rmdir(struct inode *un
 	parent = cont->parent;
 	root = cont->root;
 	sb = root->sb;
+	/*
+	 * Notify subsyses that rmdir() request comes.
+	 */
+	for_each_subsys(root, ss) {
+		if ((cont->subsys[ss->subsys_id]) && ss->pre_destroy)
+			ss->pre_destroy(ss, cont);
+	}
 
 	if (cgroup_has_css_refs(cont)) {
 		mutex_unlock(&cgroup_mutex);

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