[PATCH] cgroups: fix incorrect using rcu_dereference() in cgroup_subsys_state()

[Lai Jiangshan]

It's task->cgroups protected by RCU. and struct css_set.subsys[subsys_id]
is readonly(after init). so we don't need rcu_dereference() for
struct css_set.subsys[subsys_id].

the ways using cgroup_subsys_state() safely:

#1:
rcu_read_lock() / task_lock();
c = cgroup_subsys_state(tsk, id);
use c;
rcu_read_unlock() / task_unlock();


#2: use cgroup_lock() for _current_ task.
cgroup_lock();
c = cgroup_subsys_state(current, id);
use c;
cgroup_unlock();

Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
---
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 1164963..22901ff 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -359,10 +360,15 @@ static inline struct cgroup_subsys_state *cgroup_subsys_state(
 	return cgrp->subsys[subsys_id];
 }
 
+/* Caller must hold task_lock() or rcu_read_lock() */
 static inline struct cgroup_subsys_state *task_subsys_state(
 	struct task_struct *task, int subsys_id)
 {
-	return rcu_dereference(task->cgroups->subsys[subsys_id]);
+	/*
+	 * ->subsys[subsys_id] are read-only data, so we do not need
+	 * rcu_dereference() for it.
+	 */
+	return rcu_dereference(task->cgroups)->subsys[subsys_id];
 }
 
 static inline struct cgroup* task_cgroup(struct task_struct *task,