cgroups: remove events before destroying subsystem state objects

Events should be removed after rmdir of cgroup directory, but before
destroying subsystem state objects.  Let's take reference to cgroup
directory dentry to do that.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hioryu@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Dan Malek <dan@embeddedalley.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>

3 files changed, 8 insertions, 12 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b4f2201321cd..b8ad1ea99586 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -396,9 +396,6 @@ struct cftype {
          * closes the eventfd or on cgroup removing.
          * This callback must be implemented, if you want provide
          * notification functionality.
-         *
-         * Be careful. It can be called after destroy(), so you have
-         * to keep all nesessary data, until all events are removed.
          */
         int (*unregister_event)(struct cgroup *cgrp, struct cftype *cft,
                         struct eventfd_ctx *eventfd);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 87441fc75663..ef909a329750 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2994,6 +2994,7 @@ static void cgroup_event_remove(struct work_struct *work)
 
         eventfd_ctx_put(event->eventfd);
         kfree(event);
+        dput(cgrp->dentry);
 }
 
 /*
@@ -3114,6 +3115,13 @@ static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
                 goto fail;
         }
 
+        /*
+         * Events should be removed after rmdir of cgroup directory, but before
+         * destroying subsystem state objects. Let's take reference to cgroup
+         * directory dentry to do that.
+         */
+        dget(cgrp->dentry);
+
         spin_lock(&cgrp->event_list_lock);
         list_add(&event->list, &cgrp->event_list);
         spin_unlock(&cgrp->event_list_lock);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f9ae4b4c36eb..f7b910fc14fb 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3361,12 +3361,6 @@ static int mem_cgroup_register_event(struct cgroup *cgrp, struct cftype *cft,
                 }
         }
 
-        /*
-         * We need to increment refcnt to be sure that all thresholds
-         * will be unregistered before calling __mem_cgroup_free()
-         */
-        mem_cgroup_get(memcg);
-
         if (type == _MEM)
                 rcu_assign_pointer(memcg->thresholds, thresholds_new);
         else
@@ -3460,9 +3454,6 @@ assign:
         /* To be sure that nobody uses thresholds before freeing it */
         synchronize_rcu();
 
-        for (i = 0; i < thresholds->size - size; i++)
-                mem_cgroup_put(memcg);
-
         kfree(thresholds);
 unlock:
         mutex_unlock(&memcg->thresholds_lock);