[PATCH][rc1] cgroup: catch bad css refcnt at css_put

[KAMEZAWA Hiroyuki]

This is a patch for checking css->refcnt's sanity at css_put().

BTW, I noticed that...css->refcnt may overflow if used with memcg...
Now, refcnt is incremented per a page. Paul, do you have any idea ?
(Ah, yes. "don't use css->refcnt per page" is maybe reasonable but
 it will be big change..)

==
__css_put() doesn't check a bug as refcnt goes to minus.
I think it should be caught. This patch adds a check for it.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 kernel/cgroup.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Index: linux-2.6.32-rc1/kernel/cgroup.c
===================================================================
--- linux-2.6.32-rc1.orig/kernel/cgroup.c
+++ linux-2.6.32-rc1/kernel/cgroup.c
@@ -3708,8 +3708,10 @@ static void check_for_release(struct cgr
 void __css_put(struct cgroup_subsys_state *css)
 {
 	struct cgroup *cgrp = css->cgroup;
+	int val;
 	rcu_read_lock();
-	if (atomic_dec_return(&css->refcnt) == 1) {
+	val = atomic_dec_return(&css->refcnt);
+	if (val == 1) {
 		if (notify_on_release(cgrp)) {
 			set_bit(CGRP_RELEASABLE, &cgrp->flags);
 			check_for_release(cgrp);
@@ -3717,6 +3719,7 @@ void __css_put(struct cgroup_subsys_stat
 		cgroup_wakeup_rmdir_waiter(cgrp);
 	}
 	rcu_read_unlock();
+	WARN_ON(val < 1);
 }
 
 /*

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