cgroup: catch bad css refcnt at css_put

__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@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d2b88596efde..ca83b73fba19 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3708,8 +3708,10 @@ static void check_for_release(struct cgroup *cgrp)
 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_state *css)
                 cgroup_wakeup_rmdir_waiter(cgrp);
         }
         rcu_read_unlock();
+        WARN_ON_ONCE(val < 1);
 }
 
 /*