cgroup: fix cftype->file_offset handling

6f60eade24 ("cgroup: generalize obtaining the handles of and
notifying cgroup files") introduced cftype->file_offset so that the
handles for per-css file instances can be recorded.  These handles
then can be used, for example, to generate file modified
notifications.

Unfortunately, it made the wrong assumption that files are created
once for a given css and removed on its destruction.  Due to the
dependencies among subsystems, a css may be hidden from userland and
then later shown again.  This is implemented by removing and
re-creating the affected files, so the associated kernfs_node for a
given cgroup file may change over time.  This incorrect assumption led
to the corruption of css->files lists.

Reimplement cftype->file_offset handling so that cgroup_file->kn is
protected by a lock and updated as files are created and destroyed.
This also makes keeping them on per-cgroup list unnecessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: James Sedgwick <jsedgwick@fb.com>
Fixes: 6f60eade24 ("cgroup: generalize obtaining the handles of and notifying cgroup files")
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Zefan Li <lizefan@huawei.com>

include/linux/cgroup-defs.h

@@ -90,7 +90,6 @@ enum {
  */
 struct cgroup_file {
 	/* do not access any fields from outside cgroup core */
-	struct list_head node;			/* anchored at css->files */
 	struct kernfs_node *kn;
 };
 
@@ -134,9 +133,6 @@ struct cgroup_subsys_state {
 	 */
 	u64 serial_nr;
 
-	/* all cgroup_files associated with this css */
-	struct list_head files;
-
 	/* percpu_ref killing and RCU release */
 	struct rcu_head rcu_head;
 	struct work_struct destroy_work;