[PATCH 1/2] cgroups: enhance task_cgroup()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

task_cgroup() calls cgroup_subsys_state().
and we must use rcu_read_lock() to protect cgroup_subsys_state().
so we must use rcu_read_lock() to protect task_cgroup().

but it'll not so friendly to caller: the callers of task_cgroup() have
held cgroup_lock(). it means that struct cgroup will not be freed.

So this patch add rcu_read_lock() in task_cgroup() to enhance task_cgroup().
And we do NOT NEED FIX task_cgroup()'s callers, and cgroup_lock()
can protect task_cgroup().

Signed-off-by: Lai Jiangshan <laijs@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
@@ -359,6 +360,10 @@
 static inline struct cgroup* task_cgroup(struct task_struct *task,
 					       int subsys_id)
 {
-	return task_subsys_state(task, subsys_id)->cgroup;
+	struct cgroup *ret;
+	rcu_read_lock();
+	ret = task_subsys_state(task, subsys_id)->cgroup;
+	rcu_read_unlock();
+	return ret;
 }
 
 int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *ss,