module: move find_module check to end

I think Rusty may have made the lock a bit _too_ finegrained there, and
didn't add it to some places that needed it. It looks, for example, like
PATCH 1/2 actually drops the lock in places where it's needed
("find_module()" is documented to need it, but now load_module() didn't
hold it at all when it did the find_module()).

Rather than adding a new "module_loading" list, I think we should be able
to just use the existing "modules" list, and just fix up the locking a
bit.

In fact, maybe we could just move the "look up existing module" a bit
later - optimistically assuming that the module doesn't exist, and then
just undoing the work if it turns out that we were wrong, just before
adding ourselves to the list.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/module.c

@@ -2226,11 +2226,6 @@ static noinline struct module *load_module(void __user *umod,
 		goto free_mod;
 	}
 
-	if (find_module(mod->name)) {
-		err = -EEXIST;
-		goto free_mod;
-	}
-
 	mod->state = MODULE_STATE_COMING;
 
 	/* Allow arches to frob section contents and sizes.  */
@@ -2509,6 +2504,12 @@ static noinline struct module *load_module(void __user *umod,
 	 * The mutex protects against concurrent writers.
 	 */
 	mutex_lock(&module_mutex);
+	if (find_module(mod->name)) {
+		err = -EEXIST;
+		/* This will also unlock the mutex */
+		goto already_exists;
+	}
+
 	list_add_rcu(&mod->list, &modules);
 	mutex_unlock(&module_mutex);
 
@@ -2535,6 +2536,7 @@ static noinline struct module *load_module(void __user *umod,
 	mutex_lock(&module_mutex);
 	/* Unlink carefully: kallsyms could be walking list. */
 	list_del_rcu(&mod->list);
+ already_exists:
 	mutex_unlock(&module_mutex);
 	synchronize_sched();
 	module_arch_cleanup(mod);