memory cgroup enhancements: add- pre_destroy() handler

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

I think this is useful for subsys 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
ref to be used by other objects than tasks.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Kirill Korotaev <dev@sw.ru>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Paul Menage <menage@google.com>
Cc: Pavel Emelianov <xemul@openvz.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 87479328d46d..d8e92223a79c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -233,6 +233,7 @@ int cgroup_is_descendant(const struct cgroup *cont);
 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);