cgroup: use read lock to guard find_existing_css_set()

The function does not modify anything (except the temporary css template), so
it's sufficient to hold read lock.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup.c

@@ -415,11 +415,11 @@ static struct css_set *find_css_set(
 
 	/* First see if we already have a cgroup group that matches
 	 * the desired set */
-	write_lock(&css_set_lock);
+	read_lock(&css_set_lock);
 	res = find_existing_css_set(oldcg, cgrp, template);
 	if (res)
 		get_css_set(res);
-	write_unlock(&css_set_lock);
+	read_unlock(&css_set_lock);
 
 	if (res)
 		return res;