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memcg: fix css_is_ancestor() RCU locking

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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 <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Cc: Balbir Singh <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
KAMEZAWA Hiroyuki 2010-05-11 14:06:59 -07:00 • committed by Linus Torvalds
commit 747388d78a
2 changed files with 26 additions and 9 deletions
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31
kernel/cgroup.c
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@ -4464,15 +4464,36 @@ unsigned short css_depth(struct cgroup_subsys_state *css)
}
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)