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

@@ -5616,15 +5616,23 @@ struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id)
 }
 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)
 {
 	struct cgroup *cgrp;
 	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)

kernel/events/core.c

@@ -591,6 +591,8 @@ static inline int perf_cgroup_connect(int fd, struct perf_event *event,
 	if (!f.file)
 		return -EBADF;
 
+	rcu_read_lock();
+
 	css = cgroup_css_from_dir(f.file, perf_subsys_id);
 	if (IS_ERR(css)) {
 		ret = PTR_ERR(css);
@@ -617,6 +619,7 @@ static inline int perf_cgroup_connect(int fd, struct perf_event *event,
 		ret = -EINVAL;
 	}
 out:
+	rcu_read_unlock();
 	fdput(f);
 	return ret;
 }