[PATCH][rc1] cgroup: catch bad css refcnt at css_put v2

[KAMEZAWA Hiroyuki]

__css_put() doesn't check a buggy case as refcnt goes to minus.
This patch adds a check for it.

Changelog:
  - using WARN_ON_ONCE() instead of WARN_ON()

Acked-by: Paul Menage <menage@xxxxxxxxxx>
Acked-by: Li Zefan <lizf@xxxxxxxxxxxxxx>
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_ONCE(val < 1);
 }
 
 /*

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