memcg: fix css_is_ancestor() RCU locking
Some callers (in memcontrol.c) calls css_is_ancestor() without
rcu_read_lock. Because css_is_ancestor() has to access RCU protected
data, it should be under rcu_read_lock().
This makes css_is_ancestor() itself does safe access to RCU protected
area. (At least, "root" can have refcnt==0 if it's not an ancestor of
"child". So, we need rcu_read_lock().)
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6db8b7f..6d870f2 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4464,15 +4464,36 @@
}
EXPORT_SYMBOL_GPL(css_depth);
+/**
+ * css_is_ancestor - test "root" css is an ancestor of "child"
+ * @child: the css to be tested.
+ * @root: the css supporsed to be an ancestor of the child.
+ *
+ * Returns true if "root" is an ancestor of "child" in its hierarchy. Because
+ * this function reads css->id, this use rcu_dereference() and rcu_read_lock().
+ * But, considering usual usage, the csses should be valid objects after test.
+ * Assuming that the caller will do some action to the child if this returns
+ * returns true, the caller must take "child";s reference count.
+ * If "child" is valid object and this returns true, "root" is valid, too.
+ */
+
bool css_is_ancestor(struct cgroup_subsys_state *child,
const struct cgroup_subsys_state *root)
{
- struct css_id *child_id = rcu_dereference(child->id);
- struct css_id *root_id = rcu_dereference(root->id);
+ struct css_id *child_id;
+ struct css_id *root_id;
+ bool ret = true;
- if (!child_id || !root_id || (child_id->depth < root_id->depth))
- return false;
- return child_id->stack[root_id->depth] == root_id->id;
+ rcu_read_lock();
+ child_id = rcu_dereference(child->id);
+ root_id = rcu_dereference(root->id);
+ if (!child_id
+ || !root_id
+ || (child_id->depth < root_id->depth)
+ || (child_id->stack[root_id->depth] != root_id->id))
+ ret = false;
+ rcu_read_unlock();
+ return ret;
}
static void __free_css_id_cb(struct rcu_head *head)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 595d03f..8a79a6f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -811,12 +811,10 @@
* enabled in "curr" and "curr" is a child of "mem" in *cgroup*
* hierarchy(even if use_hierarchy is disabled in "mem").
*/
- rcu_read_lock();
if (mem->use_hierarchy)
ret = css_is_ancestor(&curr->css, &mem->css);
else
ret = (curr == mem);
- rcu_read_unlock();
css_put(&curr->css);
return ret;
}
@@ -1603,7 +1601,6 @@
* There is a small race that "from" or "to" can be
* freed by rmdir, so we use css_tryget().
*/
- rcu_read_lock();
from = mc.from;
to = mc.to;
if (from && css_tryget(&from->css)) {
@@ -1624,7 +1621,6 @@
do_continue = (to == mem_over_limit);
css_put(&to->css);
}
- rcu_read_unlock();
if (do_continue) {
DEFINE_WAIT(wait);
prepare_to_wait(&mc.waitq, &wait,