cgroup: cgroup_css_from_dir() now should be called with RCU read locked

cgroup->subsys[] will become RCU protected and thus all cgroup_css()
usages should either be under RCU read lock or cgroup_mutex.  This
patch updates cgroup_css_from_dir() which returns the matching
cgroup_subsys_state given a directory file and subsys_id so that it
requires RCU read lock and updates its sole user
perf_cgroup_connect().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>

kernel/cgroup.c

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5c6dd7e..cbb6314 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5616,8 +5616,14 @@
 }
 EXPORT_SYMBOL_GPL(css_lookup);
 
-/*
- * get corresponding css from file open on cgroupfs directory
+/**
+ * cgroup_css_from_dir - get corresponding css from file open on cgroup dir
+ * @f: directory file of interest
+ * @id: subsystem id of interest
+ *
+ * Must be called under RCU read lock.  The caller is responsible for
+ * pinning the returned css if it needs to be accessed outside the RCU
+ * critical section.
  */
 struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
 {
@@ -5625,6 +5631,8 @@
 	struct inode *inode;
 	struct cgroup_subsys_state *css;
 
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
 	inode = file_inode(f);
 	/* check in cgroup filesystem dir */
 	if (inode->i_op != &cgroup_dir_inode_operations)