TOMOYO: Simplify garbage collector.

When TOMOYO started using garbage collector at commit 847b173e "TOMOYO: Add
garbage collector.", we waited for close() before kfree(). Thus, elements to be
kfree()d were queued up using tomoyo_gc_list list.

But it turned out that tomoyo_element_linked_by_gc() tends to choke garbage
collector when certain pattern of entries are queued.

Since garbage collector is no longer waiting for close() since commit 2e503bbb
"TOMOYO: Fix lockdep warning.", we can remove tomoyo_gc_list list and
tomoyo_element_linked_by_gc() by doing sequential processing.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 169 insertions, 311 deletions

diff --git a/security/tomoyo/gc.c b/security/tomoyo/gc.c
index 7747ceb9a22..f2295c65f1e 100644
--- a/security/tomoyo/gc.c
+++ b/security/tomoyo/gc.c
@@ -13,35 +13,6 @@ static LIST_HEAD(tomoyo_io_buffer_list);
 /* Lock for protecting tomoyo_io_buffer_list. */
 static DEFINE_SPINLOCK(tomoyo_io_buffer_list_lock);
 
-/* Size of an element. */
-static const u8 tomoyo_element_size[TOMOYO_MAX_POLICY] = {
-	[TOMOYO_ID_GROUP] = sizeof(struct tomoyo_group),
-	[TOMOYO_ID_ADDRESS_GROUP] = sizeof(struct tomoyo_address_group),
-	[TOMOYO_ID_PATH_GROUP] = sizeof(struct tomoyo_path_group),
-	[TOMOYO_ID_NUMBER_GROUP] = sizeof(struct tomoyo_number_group),
-	[TOMOYO_ID_AGGREGATOR] = sizeof(struct tomoyo_aggregator),
-	[TOMOYO_ID_TRANSITION_CONTROL] =
-	sizeof(struct tomoyo_transition_control),
-	[TOMOYO_ID_MANAGER] = sizeof(struct tomoyo_manager),
-	/* [TOMOYO_ID_CONDITION] = "struct tomoyo_condition"->size, */
-	/* [TOMOYO_ID_NAME] = "struct tomoyo_name"->size, */
-	/* [TOMOYO_ID_ACL] =
-	   tomoyo_acl_size["struct tomoyo_acl_info"->type], */
-	[TOMOYO_ID_DOMAIN] = sizeof(struct tomoyo_domain_info),
-};
-
-/* Size of a domain ACL element. */
-static const u8 tomoyo_acl_size[] = {
-	[TOMOYO_TYPE_PATH_ACL] = sizeof(struct tomoyo_path_acl),
-	[TOMOYO_TYPE_PATH2_ACL] = sizeof(struct tomoyo_path2_acl),
-	[TOMOYO_TYPE_PATH_NUMBER_ACL] = sizeof(struct tomoyo_path_number_acl),
-	[TOMOYO_TYPE_MKDEV_ACL] = sizeof(struct tomoyo_mkdev_acl),
-	[TOMOYO_TYPE_MOUNT_ACL] = sizeof(struct tomoyo_mount_acl),
-	[TOMOYO_TYPE_INET_ACL] = sizeof(struct tomoyo_inet_acl),
-	[TOMOYO_TYPE_UNIX_ACL] = sizeof(struct tomoyo_unix_acl),
-	[TOMOYO_TYPE_ENV_ACL] = sizeof(struct tomoyo_env_acl),
-};
-
 /**
  * tomoyo_struct_used_by_io_buffer - Check whether the list element is used by /sys/kernel/security/tomoyo/ users or not.
  *
@@ -59,15 +30,11 @@ static bool tomoyo_struct_used_by_io_buffer(const struct list_head *element)
 	list_for_each_entry(head, &tomoyo_io_buffer_list, list) {
 		head->users++;
 		spin_unlock(&tomoyo_io_buffer_list_lock);
-		if (mutex_lock_interruptible(&head->io_sem)) {
-			in_use = true;
-			goto out;
-		}
+		mutex_lock(&head->io_sem);
 		if (head->r.domain == element || head->r.group == element ||
 		    head->r.acl == element || &head->w.domain->list == element)
 			in_use = true;
 		mutex_unlock(&head->io_sem);
-out:
 		spin_lock(&tomoyo_io_buffer_list_lock);
 		head->users--;
 		if (in_use)
@@ -81,15 +48,14 @@ out:
  * tomoyo_name_used_by_io_buffer - Check whether the string is used by /sys/kernel/security/tomoyo/ users or not.
  *
  * @string: String to check.
- * @size:   Memory allocated for @string .
  *
  * Returns true if @string is used by /sys/kernel/security/tomoyo/ users,
  * false otherwise.
  */
-static bool tomoyo_name_used_by_io_buffer(const char *string,
-					  const size_t size)
+static bool tomoyo_name_used_by_io_buffer(const char *string)
 {
 	struct tomoyo_io_buffer *head;
+	const size_t size = strlen(string) + 1;
 	bool in_use = false;
 
 	spin_lock(&tomoyo_io_buffer_list_lock);
@@ -97,10 +63,7 @@ static bool tomoyo_name_used_by_io_buffer(const char *string,
 		int i;
 		head->users++;
 		spin_unlock(&tomoyo_io_buffer_list_lock);
-		if (mutex_lock_interruptible(&head->io_sem)) {
-			in_use = true;
-			goto out;
-		}
+		mutex_lock(&head->io_sem);
 		for (i = 0; i < TOMOYO_MAX_IO_READ_QUEUE; i++) {
 			const char *w = head->r.w[i];
 			if (w < string || w > string + size)
@@ -109,7 +72,6 @@ static bool tomoyo_name_used_by_io_buffer(const char *string,
 			break;
 		}
 		mutex_unlock(&head->io_sem);
-out:
 		spin_lock(&tomoyo_io_buffer_list_lock);
 		head->users--;
 		if (in_use)
@@ -119,84 +81,6 @@ out:
 	return in_use;
 }
 
-/* Structure for garbage collection. */
-struct tomoyo_gc {
-	struct list_head list;
-	enum tomoyo_policy_id type;
-	size_t size;
-	struct list_head *element;
-};
-/* List of entries to be deleted. */
-static LIST_HEAD(tomoyo_gc_list);
-/* Length of tomoyo_gc_list. */
-static int tomoyo_gc_list_len;
-
-/**
- * tomoyo_add_to_gc - Add an entry to to be deleted list.
- *
- * @type:    One of values in "enum tomoyo_policy_id".
- * @element: Pointer to "struct list_head".
- *
- * Returns true on success, false otherwise.
- *
- * Caller holds tomoyo_policy_lock mutex.
- *
- * Adding an entry needs kmalloc(). Thus, if we try to add thousands of
- * entries at once, it will take too long time. Thus, do not add more than 128
- * entries per a scan. But to be able to handle worst case where all entries
- * are in-use, we accept one more entry per a scan.
- *
- * If we use singly linked list using "struct list_head"->prev (which is
- * LIST_POISON2), we can avoid kmalloc().
- */
-static bool tomoyo_add_to_gc(const int type, struct list_head *element)
-{
-	struct tomoyo_gc *entry = kzalloc(sizeof(*entry), GFP_ATOMIC);
-	if (!entry)
-		return false;
-	entry->type = type;
-	if (type == TOMOYO_ID_ACL)
-		entry->size = tomoyo_acl_size[
-			      container_of(element,
-					   typeof(struct tomoyo_acl_info),
-					   list)->type];
-	else if (type == TOMOYO_ID_NAME)
-		entry->size = strlen(container_of(element,
-						  typeof(struct tomoyo_name),
-						  head.list)->entry.name) + 1;
-	else if (type == TOMOYO_ID_CONDITION)
-		entry->size =
-			container_of(element, typeof(struct tomoyo_condition),
-				     head.list)->size;
-	else
-		entry->size = tomoyo_element_size[type];
-	entry->element = element;
-	list_add(&entry->list, &tomoyo_gc_list);
-	list_del_rcu(element);
-	return tomoyo_gc_list_len++ < 128;
-}
-
-/**
- * tomoyo_element_linked_by_gc - Validate next element of an entry.
- *
- * @element: Pointer to an element.
- * @size:    Size of @element in byte.
- *
- * Returns true if @element is linked by other elements in the garbage
- * collector's queue, false otherwise.
- */
-static bool tomoyo_element_linked_by_gc(const u8 *element, const size_t size)
-{
-	struct tomoyo_gc *p;
-	list_for_each_entry(p, &tomoyo_gc_list, list) {
-		const u8 *ptr = (const u8 *) p->element->next;
-		if (ptr < element || element + size < ptr)
-			continue;
-		return true;
-	}
-	return false;
-}
-
 /**
  * tomoyo_del_transition_control - Delete members in "struct tomoyo_transition_control".
  *
@@ -204,7 +88,7 @@ static bool tomoyo_element_linked_by_gc(const u8 *element, const size_t size)
  *
  * Returns nothing.
  */
-static void tomoyo_del_transition_control(struct list_head *element)
+static inline void tomoyo_del_transition_control(struct list_head *element)
 {
 	struct tomoyo_transition_control *ptr =
 		container_of(element, typeof(*ptr), head.list);
@@ -219,7 +103,7 @@ static void tomoyo_del_transition_control(struct list_head *element)
  *
  * Returns nothing.
  */
-static void tomoyo_del_aggregator(struct list_head *element)
+static inline void tomoyo_del_aggregator(struct list_head *element)
 {
 	struct tomoyo_aggregator *ptr =
 		container_of(element, typeof(*ptr), head.list);
@@ -234,7 +118,7 @@ static void tomoyo_del_aggregator(struct list_head *element)
  *
  * Returns nothing.
  */
-static void tomoyo_del_manager(struct list_head *element)
+static inline void tomoyo_del_manager(struct list_head *element)
 {
 	struct tomoyo_manager *ptr =
 		container_of(element, typeof(*ptr), head.list);
@@ -330,44 +214,24 @@ static void tomoyo_del_acl(struct list_head *element)
  *
  * @element: Pointer to "struct list_head".
  *
- * Returns true if deleted, false otherwise.
+ * Returns nothing.
  */
-static bool tomoyo_del_domain(struct list_head *element)
+static inline void tomoyo_del_domain(struct list_head *element)
 {
 	struct tomoyo_domain_info *domain =
 		container_of(element, typeof(*domain), list);
 	struct tomoyo_acl_info *acl;
 	struct tomoyo_acl_info *tmp;
 	/*
-	 * Since we don't protect whole execve() operation using SRCU,
-	 * we need to recheck domain->users at this point.
-	 *
-	 * (1) Reader starts SRCU section upon execve().
-	 * (2) Reader traverses tomoyo_domain_list and finds this domain.
-	 * (3) Writer marks this domain as deleted.
-	 * (4) Garbage collector removes this domain from tomoyo_domain_list
-	 *     because this domain is marked as deleted and used by nobody.
-	 * (5) Reader saves reference to this domain into
-	 *     "struct linux_binprm"->cred->security .
-	 * (6) Reader finishes SRCU section, although execve() operation has
-	 *     not finished yet.
-	 * (7) Garbage collector waits for SRCU synchronization.
-	 * (8) Garbage collector kfree() this domain because this domain is
-	 *     used by nobody.
-	 * (9) Reader finishes execve() operation and restores this domain from
-	 *     "struct linux_binprm"->cred->security.
-	 *
-	 * By updating domain->users at (5), we can solve this race problem
-	 * by rechecking domain->users at (8).
+	 * Since this domain is referenced from neither
+	 * "struct tomoyo_io_buffer" nor "struct cred"->security, we can delete
+	 * elements without checking for is_deleted flag.
 	 */
-	if (atomic_read(&domain->users))
-		return false;
 	list_for_each_entry_safe(acl, tmp, &domain->acl_info_list, list) {
 		tomoyo_del_acl(&acl->list);
 		tomoyo_memory_free(acl);
 	}
 	tomoyo_put_name(domain->domainname);
-	return true;
 }
 
 /**
@@ -416,10 +280,9 @@ void tomoyo_del_condition(struct list_head *element)
  *
  * Returns nothing.
  */
-static void tomoyo_del_name(struct list_head *element)
+static inline void tomoyo_del_name(struct list_head *element)
 {
-	const struct tomoyo_name *ptr =
-		container_of(element, typeof(*ptr), head.list);
+	/* Nothing to do. */
 }
 
 /**
@@ -429,7 +292,7 @@ static void tomoyo_del_name(struct list_head *element)
  *
  * Returns nothing.
  */
-static void tomoyo_del_path_group(struct list_head *element)
+static inline void tomoyo_del_path_group(struct list_head *element)
 {
 	struct tomoyo_path_group *member =
 		container_of(element, typeof(*member), head.list);
@@ -443,7 +306,7 @@ static void tomoyo_del_path_group(struct list_head *element)
  *
  * Returns nothing.
  */
-static void tomoyo_del_group(struct list_head *element)
+static inline void tomoyo_del_group(struct list_head *element)
 {
 	struct tomoyo_group *group =
 		container_of(element, typeof(*group), head.list);
@@ -469,10 +332,109 @@ static inline void tomoyo_del_address_group(struct list_head *element)
  *
  * Returns nothing.
  */
-static void tomoyo_del_number_group(struct list_head *element)
+static inline void tomoyo_del_number_group(struct list_head *element)
 {
-	struct tomoyo_number_group *member =
-		container_of(element, typeof(*member), head.list);
+	/* Nothing to do. */
+}
+
+/**
+ * tomoyo_try_to_gc - Try to kfree() an entry.
+ *
+ * @type:    One of values in "enum tomoyo_policy_id".
+ * @element: Pointer to "struct list_head".
+ *
+ * Returns nothing.
+ *
+ * Caller holds tomoyo_policy_lock mutex.
+ */
+static void tomoyo_try_to_gc(const enum tomoyo_policy_id type,
+			     struct list_head *element)
+{
+	/*
+	 * __list_del_entry() guarantees that the list element became no longer
+	 * reachable from the list which the element was originally on (e.g.
+	 * tomoyo_domain_list). Also, synchronize_srcu() guarantees that the
+	 * list element became no longer referenced by syscall users.
+	 */
+	__list_del_entry(element);
+	mutex_unlock(&tomoyo_policy_lock);
+	synchronize_srcu(&tomoyo_ss);
+	/*
+	 * However, there are two users which may still be using the list
+	 * element. We need to defer until both users forget this element.
+	 *
+	 * Don't kfree() until "struct tomoyo_io_buffer"->r.{domain,group,acl}
+	 * and "struct tomoyo_io_buffer"->w.domain forget this element.
+	 */
+	if (tomoyo_struct_used_by_io_buffer(element))
+		goto reinject;
+	switch (type) {
+	case TOMOYO_ID_TRANSITION_CONTROL:
+		tomoyo_del_transition_control(element);
+		break;
+	case TOMOYO_ID_MANAGER:
+		tomoyo_del_manager(element);
+		break;
+	case TOMOYO_ID_AGGREGATOR:
+		tomoyo_del_aggregator(element);
+		break;
+	case TOMOYO_ID_GROUP:
+		tomoyo_del_group(element);
+		break;
+	case TOMOYO_ID_PATH_GROUP:
+		tomoyo_del_path_group(element);
+		break;
+	case TOMOYO_ID_ADDRESS_GROUP:
+		tomoyo_del_address_group(element);
+		break;
+	case TOMOYO_ID_NUMBER_GROUP:
+		tomoyo_del_number_group(element);
+		break;
+	case TOMOYO_ID_CONDITION:
+		tomoyo_del_condition(element);
+		break;
+	case TOMOYO_ID_NAME:
+		/*
+		 * Don't kfree() until all "struct tomoyo_io_buffer"->r.w[]
+		 * forget this element.
+		 */
+		if (tomoyo_name_used_by_io_buffer
+		    (container_of(element, typeof(struct tomoyo_name),
+				  head.list)->entry.name))
+			goto reinject;
+		tomoyo_del_name(element);
+		break;
+	case TOMOYO_ID_ACL:
+		tomoyo_del_acl(element);
+		break;
+	case TOMOYO_ID_DOMAIN:
+		/*
+		 * Don't kfree() until all "struct cred"->security forget this
+		 * element.
+		 */
+		if (atomic_read(&container_of
+				(element, typeof(struct tomoyo_domain_info),
+				 list)->users))
+			goto reinject;
+		tomoyo_del_domain(element);
+		break;
+	case TOMOYO_MAX_POLICY:
+		break;
+	}
+	mutex_lock(&tomoyo_policy_lock);
+	tomoyo_memory_free(element);
+	return;
+reinject:
+	/*
+	 * We can safely reinject this element here bacause
+	 * (1) Appending list elements and removing list elements are protected
+	 *     by tomoyo_policy_lock mutex.
+	 * (2) Only this function removes list elements and this function is
+	 *     exclusively executed by tomoyo_gc_mutex mutex.
+	 * are true.
+	 */
+	mutex_lock(&tomoyo_policy_lock);
+	list_add_rcu(element, element->prev);
 }
 
 /**
@@ -481,19 +443,19 @@ static void tomoyo_del_number_group(struct list_head *element)
  * @id:          One of values in "enum tomoyo_policy_id".
  * @member_list: Pointer to "struct list_head".
  *
- * Returns true if some elements are deleted, false otherwise.
+ * Returns nothing.
  */
-static bool tomoyo_collect_member(const enum tomoyo_policy_id id,
+static void tomoyo_collect_member(const enum tomoyo_policy_id id,
 				  struct list_head *member_list)
 {
 	struct tomoyo_acl_head *member;
-	list_for_each_entry(member, member_list, list) {
+	struct tomoyo_acl_head *tmp;
+	list_for_each_entry_safe(member, tmp, member_list, list) {
 		if (!member->is_deleted)
 			continue;
-		if (!tomoyo_add_to_gc(id, &member->list))
-			return false;
+		member->is_deleted = TOMOYO_GC_IN_PROGRESS;
+		tomoyo_try_to_gc(id, &member->list);
 	}
-	return true;
 }
 
 /**
@@ -501,22 +463,22 @@ static bool tomoyo_collect_member(const enum tomoyo_policy_id id,
  *
  * @list: Pointer to "struct list_head".
  *
- * Returns true if some elements are deleted, false otherwise.
+ * Returns nothing.
  */
-static bool tomoyo_collect_acl(struct list_head *list)
+static void tomoyo_collect_acl(struct list_head *list)
 {
 	struct tomoyo_acl_info *acl;
-	list_for_each_entry(acl, list, list) {
+	struct tomoyo_acl_info *tmp;
+	list_for_each_entry_safe(acl, tmp, list, list) {
 		if (!acl->is_deleted)
 			continue;
-		if (!tomoyo_add_to_gc(TOMOYO_ID_ACL, &acl->list))
-			return false;
+		acl->is_deleted = TOMOYO_GC_IN_PROGRESS;
+		tomoyo_try_to_gc(TOMOYO_ID_ACL, &acl->list);
 	}
-	return true;
 }
 
 /**
- * tomoyo_collect_entry - Scan lists for deleted elements.
+ * tomoyo_collect_entry - Try to kfree() deleted elements.
  *
  * Returns nothing.
  */
@@ -525,36 +487,40 @@ static void tomoyo_collect_entry(void)
 	int i;
 	enum tomoyo_policy_id id;
 	struct tomoyo_policy_namespace *ns;
-	int idx;
-	if (mutex_lock_interruptible(&tomoyo_policy_lock))
-		return;
-	idx = tomoyo_read_lock();
+	mutex_lock(&tomoyo_policy_lock);
 	{
 		struct tomoyo_domain_info *domain;
-		list_for_each_entry_rcu(domain, &tomoyo_domain_list, list) {
-			if (!tomoyo_collect_acl(&domain->acl_info_list))
-				goto unlock;
+		struct tomoyo_domain_info *tmp;
+		list_for_each_entry_safe(domain, tmp, &tomoyo_domain_list,
+					 list) {
+			tomoyo_collect_acl(&domain->acl_info_list);
 			if (!domain->is_deleted || atomic_read(&domain->users))
 				continue;
-			/*
-			 * Nobody is referring this domain. But somebody may
-			 * refer this domain after successful execve().
-			 * We recheck domain->users after SRCU synchronization.
-			 */
-			if (!tomoyo_add_to_gc(TOMOYO_ID_DOMAIN, &domain->list))
-				goto unlock;
+			tomoyo_try_to_gc(TOMOYO_ID_DOMAIN, &domain->list);
 		}
 	}
-	list_for_each_entry_rcu(ns, &tomoyo_namespace_list, namespace_list) {
+	list_for_each_entry(ns, &tomoyo_namespace_list, namespace_list) {
 		for (id = 0; id < TOMOYO_MAX_POLICY; id++)
-			if (!tomoyo_collect_member(id, &ns->policy_list[id]))
-				goto unlock;
+			tomoyo_collect_member(id, &ns->policy_list[id]);
 		for (i = 0; i < TOMOYO_MAX_ACL_GROUPS; i++)
-			if (!tomoyo_collect_acl(&ns->acl_group[i]))
-				goto unlock;
+			tomoyo_collect_acl(&ns->acl_group[i]);
+	}
+	{
+		struct tomoyo_shared_acl_head *ptr;
+		struct tomoyo_shared_acl_head *tmp;
+		list_for_each_entry_safe(ptr, tmp, &tomoyo_condition_list,
+					 list) {
+			if (atomic_read(&ptr->users) > 0)
+				continue;
+			atomic_set(&ptr->users, TOMOYO_GC_IN_PROGRESS);
+			tomoyo_try_to_gc(TOMOYO_ID_CONDITION, &ptr->list);
+		}
+	}
+	list_for_each_entry(ns, &tomoyo_namespace_list, namespace_list) {
 		for (i = 0; i < TOMOYO_MAX_GROUP; i++) {
 			struct list_head *list = &ns->group_list[i];
 			struct tomoyo_group *group;
+			struct tomoyo_group *tmp;
 			switch (i) {
 			case 0:
 				id = TOMOYO_ID_PATH_GROUP;
@@ -566,139 +532,37 @@ static void tomoyo_collect_entry(void)
 				id = TOMOYO_ID_ADDRESS_GROUP;
 				break;
 			}
-			list_for_each_entry(group, list, head.list) {
-				if (!tomoyo_collect_member
-				    (id, &group->member_list))
-					goto unlock;
+			list_for_each_entry_safe(group, tmp, list, head.list) {
+				tomoyo_collect_member(id, &group->member_list);
 				if (!list_empty(&group->member_list) ||
-				    atomic_read(&group->head.users))
+				    atomic_read(&group->head.users) > 0)
 					continue;
-				if (!tomoyo_add_to_gc(TOMOYO_ID_GROUP,
-						      &group->head.list))
-					goto unlock;
+				atomic_set(&group->head.users,
+					   TOMOYO_GC_IN_PROGRESS);
+				tomoyo_try_to_gc(TOMOYO_ID_GROUP,
+						 &group->head.list);
 			}
 		}
 	}
-	id = TOMOYO_ID_CONDITION;
-	for (i = 0; i < TOMOYO_MAX_HASH + 1; i++) {
-		struct list_head *list = !i ?
-			&tomoyo_condition_list : &tomoyo_name_list[i - 1];
+	for (i = 0; i < TOMOYO_MAX_HASH; i++) {
+		struct list_head *list = &tomoyo_name_list[i];
 		struct tomoyo_shared_acl_head *ptr;
-		list_for_each_entry(ptr, list, list) {
-			if (atomic_read(&ptr->users))
+		struct tomoyo_shared_acl_head *tmp;
+		list_for_each_entry_safe(ptr, tmp, list, list) {
+			if (atomic_read(&ptr->users) > 0)
 				continue;
-			if (!tomoyo_add_to_gc(id, &ptr->list))
-				goto unlock;
+			atomic_set(&ptr->users, TOMOYO_GC_IN_PROGRESS);
+			tomoyo_try_to_gc(TOMOYO_ID_NAME, &ptr->list);
 		}
-		id = TOMOYO_ID_NAME;
 	}
-unlock:
-	tomoyo_read_unlock(idx);
 	mutex_unlock(&tomoyo_policy_lock);
 }
 
 /**
- * tomoyo_kfree_entry - Delete entries in tomoyo_gc_list.
- *
- * Returns true if some entries were kfree()d, false otherwise.
- */
-static bool tomoyo_kfree_entry(void)
-{
-	struct tomoyo_gc *p;
-	struct tomoyo_gc *tmp;
-	bool result = false;
-
-	list_for_each_entry_safe(p, tmp, &tomoyo_gc_list, list) {
-		struct list_head *element = p->element;
-
-		/*
-		 * list_del_rcu() in tomoyo_add_to_gc() guarantees that the
-		 * list element became no longer reachable from the list which
-		 * the element was originally on (e.g. tomoyo_domain_list).
-		 * Also, synchronize_srcu() in tomoyo_gc_thread() guarantees
-		 * that the list element became no longer referenced by syscall
-		 * users.
-		 *
-		 * However, there are three users which may still be using the
-		 * list element. We need to defer until all of these users
-		 * forget the list element.
-		 *
-		 * Firstly, defer until "struct tomoyo_io_buffer"->r.{domain,
-		 * group,acl} and "struct tomoyo_io_buffer"->w.domain forget
-		 * the list element.
-		 */
-		if (tomoyo_struct_used_by_io_buffer(element))
-			continue;
-		/*
-		 * Secondly, defer until all other elements in the
-		 * tomoyo_gc_list list forget the list element.
-		 */
-		if (tomoyo_element_linked_by_gc((const u8 *) element, p->size))
-			continue;
-		switch (p->type) {
-		case TOMOYO_ID_TRANSITION_CONTROL:
-			tomoyo_del_transition_control(element);
-			break;
-		case TOMOYO_ID_AGGREGATOR:
-			tomoyo_del_aggregator(element);
-			break;
-		case TOMOYO_ID_MANAGER:
-			tomoyo_del_manager(element);
-			break;
-		case TOMOYO_ID_CONDITION:
-			tomoyo_del_condition(element);
-			break;
-		case TOMOYO_ID_NAME:
-			/*
-			 * Thirdly, defer until all "struct tomoyo_io_buffer"
-			 * ->r.w[] forget the list element.
-			 */
-			if (tomoyo_name_used_by_io_buffer(
-			    container_of(element, typeof(struct tomoyo_name),
-					 head.list)->entry.name, p->size))
-				continue;
-			tomoyo_del_name(element);
-			break;
-		case TOMOYO_ID_ACL:
-			tomoyo_del_acl(element);
-			break;
-		case TOMOYO_ID_DOMAIN:
-			if (!tomoyo_del_domain(element))
-				continue;
-			break;
-		case TOMOYO_ID_PATH_GROUP:
-			tomoyo_del_path_group(element);
-			break;
-		case TOMOYO_ID_ADDRESS_GROUP:
-			tomoyo_del_address_group(element);
-			break;
-		case TOMOYO_ID_GROUP:
-			tomoyo_del_group(element);
-			break;
-		case TOMOYO_ID_NUMBER_GROUP:
-			tomoyo_del_number_group(element);
-			break;
-		case TOMOYO_MAX_POLICY:
-			break;
-		}
-		tomoyo_memory_free(element);
-		list_del(&p->list);
-		kfree(p);
-		tomoyo_gc_list_len--;
-		result = true;
-	}
-	return result;
-}
-
-/**
  * tomoyo_gc_thread - Garbage collector thread function.
  *
  * @unused: Unused.
  *
- * In case OOM-killer choose this thread for termination, we create this thread
- * as a short live thread whenever /sys/kernel/security/tomoyo/ interface was
- * close()d.
- *
  * Returns 0.
  */
 static int tomoyo_gc_thread(void *unused)
@@ -707,13 +571,7 @@ static int tomoyo_gc_thread(void *unused)
 	static DEFINE_MUTEX(tomoyo_gc_mutex);
 	if (!mutex_trylock(&tomoyo_gc_mutex))
 		goto out;
-
-	do {
-		tomoyo_collect_entry();
-		if (list_empty(&tomoyo_gc_list))
-			break;
-		synchronize_srcu(&tomoyo_ss);
-	} while (tomoyo_kfree_entry());
+	tomoyo_collect_entry();
 	{
 		struct tomoyo_io_buffer *head;
 		struct tomoyo_io_buffer *tmp;