ftrace: safe traversal of ftrace_hash hlist

Hi Steven,

I noticed that concurrent instances of ftrace_record_ip()
have a race between ftrace_hash list traversal during
ftrace_ip_in_hash() (before acquiring ftrace_shutdown_lock)
and ftrace_add_hash(). If it's so then this should fix it.
Signed-off-by: NAbhishek Sagar <sagar.abhishek@gmail.com>
Cc: rostedt@goodmis.org
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

kernel/trace/ftrace.c

@@ -201,7 +201,7 @@ ftrace_ip_in_hash(unsigned long ip, unsigned long key)
 	struct hlist_node *t;
 	int found = 0;
 
-	hlist_for_each_entry(p, t, &ftrace_hash[key], node) {
+	hlist_for_each_entry_rcu(p, t, &ftrace_hash[key], node) {
 		if (p->ip == ip) {
 			found = 1;
 			break;
@@ -214,7 +214,7 @@ ftrace_ip_in_hash(unsigned long ip, unsigned long key)
 static inline void
 ftrace_add_hash(struct dyn_ftrace *node, unsigned long key)
 {
-	hlist_add_head(&node->node, &ftrace_hash[key]);
+	hlist_add_head_rcu(&node->node, &ftrace_hash[key]);
 }
 
 static void ftrace_free_rec(struct dyn_ftrace *rec)