ftrace: Add freeing algorithm to free ftrace_mod_maps

The ftrace_mod_map is a descriptor to save module init function names in
case they were traced, and the trace output needs to reference the function
name from the function address. But after the function is unloaded, it
the maps should be freed, as the rest of the function names are as well.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -5683,6 +5683,7 @@ struct ftrace_mod_func {
 };
 
 struct ftrace_mod_map {
+	struct rcu_head		rcu;
 	struct list_head	list;
 	struct module		*mod;
 	unsigned long		start_addr;
@@ -5694,6 +5695,8 @@ struct ftrace_mod_map {
 
 #define next_to_ftrace_page(p) container_of(p, struct ftrace_page, next)
 
+static LIST_HEAD(ftrace_mod_maps);
+
 static int referenced_filters(struct dyn_ftrace *rec)
 {
 	struct ftrace_ops *ops;
@@ -5747,8 +5750,26 @@ static void clear_mod_from_hashes(struct ftrace_page *pg)
 	mutex_unlock(&trace_types_lock);
 }
 
+static void ftrace_free_mod_map(struct rcu_head *rcu)
+{
+	struct ftrace_mod_map *mod_map = container_of(rcu, struct ftrace_mod_map, rcu);
+	struct ftrace_mod_func *mod_func;
+	struct ftrace_mod_func *n;
+
+	/* All the contents of mod_map are now not visible to readers */
+	list_for_each_entry_safe(mod_func, n, &mod_map->funcs, list) {
+		kfree(mod_func->name);
+		list_del(&mod_func->list);
+		kfree(mod_func);
+	}
+
+	kfree(mod_map);
+}
+
 void ftrace_release_mod(struct module *mod)
 {
+	struct ftrace_mod_map *mod_map;
+	struct ftrace_mod_map *n;
 	struct dyn_ftrace *rec;
 	struct ftrace_page **last_pg;
 	struct ftrace_page *tmp_page = NULL;
@@ -5760,6 +5781,14 @@ void ftrace_release_mod(struct module *mod)
 	if (ftrace_disabled)
 		goto out_unlock;
 
+	list_for_each_entry_safe(mod_map, n, &ftrace_mod_maps, list) {
+		if (mod_map->mod == mod) {
+			list_del_rcu(&mod_map->list);
+			call_rcu_sched(&mod_map->rcu, ftrace_free_mod_map);
+			break;
+		}
+	}
+
 	/*
 	 * Each module has its own ftrace_pages, remove
 	 * them from the list.
@@ -5914,8 +5943,6 @@ static void save_ftrace_mod_rec(struct ftrace_mod_map *mod_map,
 	list_add_rcu(&mod_func->list, &mod_map->funcs);
 }
 
-static LIST_HEAD(ftrace_mod_maps);
-
 static struct ftrace_mod_map *
 allocate_ftrace_mod_map(struct module *mod,
 			unsigned long start, unsigned long end)
@@ -5974,6 +6001,7 @@ ftrace_mod_address_lookup(unsigned long addr, unsigned long *size,
 	struct ftrace_mod_map *mod_map;
 	const char *ret = NULL;
 
+	/* mod_map is freed via call_rcu_sched() */
 	preempt_disable();
 	list_for_each_entry_rcu(mod_map, &ftrace_mod_maps, list) {
 		ret = ftrace_func_address_lookup(mod_map, addr, size, off, sym);