module: Fix a few concurrent module loading races

Concurrently starting multiple OSTs on a single OSS frequently
triggers 30s deadlocks on module_mutex.  This RHEL 6 kernel bug
applies to any module that results in additional request_module()
calls in its init callback.  In Lustre, at least ptlrpc and libcfs are
affected.  This patch adapts fixes from the following upstream
commits:

  9bea7f2 module: fix bne2 "gave up waiting for init of module
		  libcrc32c"
  be593f4 module: verify_export_symbols under the lock
  3bafeb6 module: move find_module check to end
  80a3d1b module: move sysfs exposure to end of load_module

diff -rNup linux-2.6.32-431.20.3.el6.orig/kernel/module.c linux-2.6.32-431.20.3.el6/kernel/module.c
--- linux-2.6.32-431.20.3.el6.orig/kernel/module.c	2014-06-07 05:42:39.000000000 +0800
+++ linux-2.6.32-431.20.3.el6/kernel/module.c	2014-07-28 00:04:43.000000000 +0800
@@ -658,6 +658,33 @@ static int already_uses(struct module *a
 }
 
 /* Module a uses b */
+static int ref_module(struct module *a, struct module *b)
+{
+	struct module_use *use;
+	int err;
+
+	if (b == NULL || already_uses(a, b))
+		return 0;
+
+	/* If module isn't available, we fail. */
+	err = strong_try_module_get(b);
+	if (err)
+		return err;
+
+	DEBUGP("Allocating new usage for %s.\n", a->name);
+	use = kmalloc(sizeof(*use), GFP_ATOMIC);
+	if (!use) {
+		printk("%s: out of memory loading\n", a->name);
+		module_put(b);
+		return -ENOMEM;
+	}
+
+	use->module_which_uses = a;
+	list_add(&use->list, &b->modules_which_use_me);
+	return 0;
+}
+
+/* Module a uses b */
 int use_module(struct module *a, struct module *b)
 {
 	struct module_use *use;
@@ -707,7 +734,6 @@ static void module_unload_free(struct mo
 				module_put(i);
 				list_del(&use->list);
 				kfree(use);
-				sysfs_remove_link(i->holders_dir, mod->name);
 				/* There can be at most one match. */
 				break;
 			}
@@ -962,6 +988,11 @@ static inline void module_unload_free(st
 {
 }
 
+static inline int ref_module(struct module *a, struct module *b)
+{
+	return strong_try_module_get(b);
+}
+
 int use_module(struct module *a, struct module *b)
 {
 	return strong_try_module_get(b) == 0;
@@ -1130,24 +1161,60 @@ static inline int same_magic(const char 
 static const struct kernel_symbol *resolve_symbol(Elf_Shdr *sechdrs,
 						  unsigned int versindex,
 						  const char *name,
-						  struct module *mod)
+						  struct module *mod,
+						  char ownername[])
 {
 	struct module *owner;
 	const struct kernel_symbol *sym;
 	const unsigned long *crc;
+	int err;
 
+	mutex_lock(&module_mutex);
 	sym = find_symbol(name, &owner, &crc,
 			  !(mod->taints & (1 << TAINT_PROPRIETARY_MODULE)), true);
-	/* use_module can fail due to OOM,
-	   or module initialization or unloading */
-	if (sym) {
-		if (!check_version(sechdrs, versindex, name, mod, crc, owner)
-		    || !use_module(mod, owner))
-			sym = NULL;
+	if (!sym)
+		goto unlock;
+
+	if (!check_version(sechdrs, versindex, name, mod, crc, owner)) {
+		sym = ERR_PTR(-EINVAL);
+		goto getname;
 	}
+
+	err = ref_module(mod, owner);
+	if (err) {
+		sym = ERR_PTR(err);
+		goto getname;
+	}
+
+getname:
+	/* We must make copy under the lock if we failed to get ref. */
+	strncpy(ownername, module_name(owner), MODULE_NAME_LEN);
+unlock:
+	mutex_unlock(&module_mutex);
 	return sym;
 }
 
+static const struct kernel_symbol *resolve_symbol_wait(Elf_Shdr *sechdrs,
+						       unsigned int versindex,
+						       const char *name,
+						       struct module *mod)
+{
+	const struct kernel_symbol *ksym;
+	char ownername[MODULE_NAME_LEN];
+
+	mutex_unlock(&module_mutex);
+	if (wait_event_interruptible_timeout(module_wq,
+			!IS_ERR(ksym = resolve_symbol(sechdrs, versindex, name,
+						      mod, ownername)) ||
+			PTR_ERR(ksym) != -EBUSY,
+				msecs_to_jiffies(30 * MSEC_PER_SEC)) <= 0) {
+		printk(KERN_WARNING "%s: gave up waiting for init of module %s.\n",
+		       mod->name, ownername);
+	}
+	mutex_lock(&module_mutex);
+	return ksym;
+}
+
 /*
  * /sys/module/foo/sections stuff
  * J. Corbet <corbet@lwn.net>
@@ -1375,6 +1442,30 @@ static inline void remove_notes_attrs(st
 #endif
 
 #ifdef CONFIG_SYSFS
+static void add_usage_links(struct module *mod)
+{
+#ifdef CONFIG_MODULE_UNLOAD
+	struct module_use *use;
+	int nowarn;
+
+	list_for_each_entry(use, &mod->modules_which_use_me, list) {
+		nowarn = sysfs_create_link(use->module_which_uses->holders_dir,
+					   &mod->mkobj.kobj, mod->name);
+	}
+#endif
+}
+
+static void del_usage_links(struct module *mod)
+{
+#ifdef CONFIG_MODULE_UNLOAD
+	struct module_use *use;
+
+	list_for_each_entry(use, &mod->modules_which_use_me, list)
+		sysfs_remove_link(use->module_which_uses->holders_dir, mod->name);
+#endif
+}
+
+
 int module_add_modinfo_attrs(struct module *mod)
 {
 	struct module_attribute *attr;
@@ -1456,6 +1547,10 @@ int mod_sysfs_setup(struct module *mod,
 {
 	int err;
 
+	err = mod_sysfs_init(mod);
+	if (err)
+		goto out;
+
 	mod->holders_dir = kobject_create_and_add("holders", &mod->mkobj.kobj);
 	if (!mod->holders_dir) {
 		err = -ENOMEM;
@@ -1470,6 +1565,8 @@ int mod_sysfs_setup(struct module *mod,
 	if (err)
 		goto out_unreg_param;
 
+	add_usage_links(mod);
+
 	kobject_uevent(&mod->mkobj.kobj, KOBJ_ADD);
 	return 0;
 
@@ -1479,6 +1576,7 @@ out_unreg_holders:
 	kobject_put(mod->holders_dir);
 out_unreg:
 	kobject_put(&mod->mkobj.kobj);
+out:
 	return err;
 }
 
@@ -1493,10 +1591,15 @@ static void mod_sysfs_fini(struct module
 {
 }
 
+static void del_usage_links(struct module *mod)
+{
+}
+
 #endif /* CONFIG_SYSFS */
 
 static void mod_kobject_remove(struct module *mod)
 {
+	del_usage_links(mod);
 	module_remove_modinfo_attrs(mod);
 	module_param_sysfs_remove(mod);
 	kobject_put(mod->mkobj.drivers_dir);
@@ -1576,6 +1679,8 @@ EXPORT_SYMBOL_GPL(__symbol_get);
 /*
  * Ensure that an exported symbol [global namespace] does not already exist
  * in the kernel or in some other module's exported symbol table.
+ *
+ * You must hold the module_mutex.
  */
 static int verify_export_symbols(struct module *mod)
 {
@@ -1641,21 +1746,23 @@ static int simplify_symbols(Elf_Shdr *se
 			break;
 
 		case SHN_UNDEF:
-			ksym = resolve_symbol(sechdrs, versindex,
-					      strtab + sym[i].st_name, mod);
+			ksym = resolve_symbol_wait(sechdrs, versindex,
+						   strtab + sym[i].st_name,
+						   mod);
 			/* Ok if resolved.  */
-			if (ksym) {
+			if (ksym && !IS_ERR(ksym)) {
 				sym[i].st_value = ksym->value;
 				break;
 			}
 
 			/* Ok if weak.  */
-			if (ELF_ST_BIND(sym[i].st_info) == STB_WEAK)
+			if (!ksym && ELF_ST_BIND(sym[i].st_info) == STB_WEAK)
 				break;
 
-			printk(KERN_WARNING "%s: Unknown symbol %s\n",
-			       mod->name, strtab + sym[i].st_name);
-			ret = -ENOENT;
+			printk(KERN_WARNING "%s: Unknown symbol %s (err %li)\n",
+			       mod->name, strtab + sym[i].st_name,
+			       PTR_ERR(ksym));
+			ret = PTR_ERR(ksym) ?: -ENOENT;
 			break;
 
 		default:
@@ -2240,11 +2347,6 @@ static noinline struct module *load_modu
 		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.  */
@@ -2338,11 +2440,6 @@ static noinline struct module *load_modu
 	/* Now we've moved module, initialize linked lists, etc. */
 	module_unload_init(mod);
 
-	/* add kobject, so we can reference it. */
-	err = mod_sysfs_init(mod);
-	if (err)
-		goto free_unload;
-
 	/* Set up license info based on the info section */
 	set_license(mod, get_modinfo(sechdrs, infoindex, "license"));
 
@@ -2461,11 +2558,6 @@ static noinline struct module *load_modu
 			goto cleanup;
 	}
 
-        /* Find duplicate symbols */
-	err = verify_export_symbols(mod);
-	if (err < 0)
-		goto cleanup;
-
   	/* Set up and sort exception table */
 	mod->extable = section_objs(hdr, sechdrs, secstrings, "__ex_table",
 				    sizeof(*mod->extable), &mod->num_exentries);
@@ -2521,6 +2613,16 @@ static noinline struct module *load_modu
 	 * function to insert in a way safe to concurrent readers.
 	 * The mutex protects against concurrent writers.
 	 */
+	if (find_module(mod->name)) {
+		err = -EEXIST;
+		goto already_exists;
+	}
+
+        /* Find duplicate symbols */
+	err = verify_export_symbols(mod);
+	if (err < 0)
+		goto already_exists;
+
 	list_add_rcu(&mod->list, &modules);
 
 	err = parse_args(mod->name, mod->args, mod->kp, mod->num_kp, NULL);
@@ -2530,6 +2632,7 @@ static noinline struct module *load_modu
 	err = mod_sysfs_setup(mod, mod->kp, mod->num_kp);
 	if (err < 0)
 		goto unlink;
+
 	add_sect_attrs(mod, hdr->e_shnum, secstrings, sechdrs);
 	add_notes_attrs(mod, hdr->e_shnum, secstrings, sechdrs);
 
@@ -2544,15 +2647,13 @@ static noinline struct module *load_modu
  unlink:
 	/* Unlink carefully: kallsyms could be walking list. */
 	list_del_rcu(&mod->list);
+ already_exists:
 	synchronize_sched();
 	module_arch_cleanup(mod);
  ddebug:
 	dynamic_debug_remove(debug);
  cleanup:
 	free_modinfo(mod);
-	kobject_del(&mod->mkobj.kobj);
-	kobject_put(&mod->mkobj.kobj);
- free_unload:
 	module_unload_free(mod);
 #if defined(CONFIG_MODULE_UNLOAD) && defined(CONFIG_SMP)
 	percpu_modfree(mod->refptr);