module: put modules in list much earlier.

Prarit's excellent bug report:
> In recent Fedora releases (F17 & F18) some users have reported seeing
> messages similar to
>
> [   15.478160] kvm: Could not allocate 304 bytes percpu data
> [   15.478174] PERCPU: allocation failed, size=304 align=32, alloc from
> reserved chunk failed
>
> during system boot.  In some cases, users have also reported seeing this
> message along with a failed load of other modules.
>
> What is happening is systemd is loading an instance of the kvm module for
> each cpu found (see commit e9bda3b3).  When the module load occurs the kernel
> currently allocates the modules percpu data area prior to checking to see
> if the module is already loaded or is in the process of being loaded.  If
> the module is already loaded, or finishes load, the module loading code
> releases the current instance's module's percpu data.

Now we have a new state MODULE_STATE_UNFORMED, we can insert the
module into the list (and thus guarantee its uniqueness) before we
allocate the per-cpu region.
Reported-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Tested-by: Prarit Bhargava <prarit@redhat.com>

kernel/module.c

@@ -3017,7 +3017,7 @@ static bool finished_loading(const char *name)
 	bool ret;
 
 	mutex_lock(&module_mutex);
-	mod = find_module(name);
+	mod = find_module_all(name, true);
 	ret = !mod || mod->state == MODULE_STATE_LIVE
 		|| mod->state == MODULE_STATE_GOING;
 	mutex_unlock(&module_mutex);
@@ -3141,6 +3141,32 @@ static int load_module(struct load_info *info, const char __user *uargs,
 		goto free_copy;
 	}
 
+	/*
+	 * We try to place it in the list now to make sure it's unique
+	 * before we dedicate too many resources.  In particular,
+	 * temporary percpu memory exhaustion.
+	 */
+	mod->state = MODULE_STATE_UNFORMED;
+again:
+	mutex_lock(&module_mutex);
+	if ((old = find_module_all(mod->name, true)) != NULL) {
+		if (old->state == MODULE_STATE_COMING
+		    || old->state == MODULE_STATE_UNFORMED) {
+			/* Wait in case it fails to load. */
+			mutex_unlock(&module_mutex);
+			err = wait_event_interruptible(module_wq,
+					       finished_loading(mod->name));
+			if (err)
+				goto free_module;
+			goto again;
+		}
+		err = -EEXIST;
+		mutex_unlock(&module_mutex);
+		goto free_module;
+	}
+	list_add_rcu(&mod->list, &modules);
+	mutex_unlock(&module_mutex);
+
 #ifdef CONFIG_MODULE_SIG
 	mod->sig_ok = info->sig_ok;
 	if (!mod->sig_ok)
@@ -3150,7 +3176,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	/* Now module is in final location, initialize linked lists, etc. */
 	err = module_unload_init(mod);
 	if (err)
-		goto free_module;
+		goto unlink_mod;
 
 	/* Now we've got everything in the final locations, we can
 	 * find optional sections. */
@@ -3185,54 +3211,33 @@ static int load_module(struct load_info *info, const char __user *uargs,
 		goto free_arch_cleanup;
 	}
 
-	/* Mark state as coming so strong_try_module_get() ignores us. */
-	mod->state = MODULE_STATE_COMING;
-
-	/* Now sew it into the lists so we can get lockdep and oops
-	 * info during argument parsing.  No one should access us, since
-	 * strong_try_module_get() will fail.
-	 * lockdep/oops can run asynchronous, so use the RCU list insertion
-	 * function to insert in a way safe to concurrent readers.
-	 * The mutex protects against concurrent writers.
-	 */
-again:
-	mutex_lock(&module_mutex);
-	if ((old = find_module(mod->name)) != NULL) {
-		if (old->state == MODULE_STATE_COMING) {
-			/* Wait in case it fails to load. */
-			mutex_unlock(&module_mutex);
-			err = wait_event_interruptible(module_wq,
-					       finished_loading(mod->name));
-			if (err)
-				goto free_arch_cleanup;
-			goto again;
-		}
-		err = -EEXIST;
-		goto unlock;
-	}
-
-	/* This has to be done once we're sure module name is unique. */
 	dynamic_debug_setup(info->debug, info->num_debug);
 
-	/* Find duplicate symbols */
+	mutex_lock(&module_mutex);
+	/* Find duplicate symbols (must be called under lock). */
 	err = verify_export_symbols(mod);
 	if (err < 0)
-		goto ddebug;
+		goto ddebug_cleanup;
 
+	/* This relies on module_mutex for list integrity. */
 	module_bug_finalize(info->hdr, info->sechdrs, mod);
-	list_add_rcu(&mod->list, &modules);
+
+	/* Mark state as coming so strong_try_module_get() ignores us,
+	 * but kallsyms etc. can see us. */
+	mod->state = MODULE_STATE_COMING;
+
 	mutex_unlock(&module_mutex);
 
 	/* Module is ready to execute: parsing args may do that. */
 	err = parse_args(mod->name, mod->args, mod->kp, mod->num_kp,
 			 -32768, 32767, &ddebug_dyndbg_module_param_cb);
 	if (err < 0)
-		goto unlink;
+		goto bug_cleanup;
 
 	/* Link in to syfs. */
 	err = mod_sysfs_setup(mod, info, mod->kp, mod->num_kp);
 	if (err < 0)
-		goto unlink;
+		goto bug_cleanup;
 
 	/* Get rid of temporary copy. */
 	free_copy(info);
@@ -3242,16 +3247,13 @@ static int load_module(struct load_info *info, const char __user *uargs,
 
 	return do_init_module(mod);
 
- unlink:
+ bug_cleanup:
+	/* module_bug_cleanup needs module_mutex protection */
 	mutex_lock(&module_mutex);
-	/* Unlink carefully: kallsyms could be walking list. */
-	list_del_rcu(&mod->list);
 	module_bug_cleanup(mod);
-	wake_up_all(&module_wq);
- ddebug:
-	dynamic_debug_remove(info->debug);
- unlock:
 	mutex_unlock(&module_mutex);
+ ddebug_cleanup:
+	dynamic_debug_remove(info->debug);
 	synchronize_sched();
 	kfree(mod->args);
  free_arch_cleanup:
@@ -3260,6 +3262,12 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	free_modinfo(mod);
  free_unload:
 	module_unload_free(mod);
+ unlink_mod:
+	mutex_lock(&module_mutex);
+	/* Unlink carefully: kallsyms could be walking list. */
+	list_del_rcu(&mod->list);
+	wake_up_all(&module_wq);
+	mutex_unlock(&module_mutex);
  free_module:
 	module_deallocate(mod, info);
  free_copy:

lib/bug.c

@@ -55,6 +55,7 @@ static inline unsigned long bug_addr(const struct bug_entry *bug)
 }
 
 #ifdef CONFIG_MODULES
+/* Updates are protected by module mutex */
 static LIST_HEAD(module_bug_list);
 
 static const struct bug_entry *module_find_bug(unsigned long bugaddr)