lockdep: Fix the module unload key range freeing logic

Module unload calls lockdep_free_key_range(), which removes entries
from the data structures. Most of the lockdep code OTOH assumes the
data structures are append only; in specific see the comments in
add_lock_to_list() and look_up_lock_class().

Clearly this has only worked by accident; make it work proper. The
actual scenario to make it go boom would involve the memory freed by
the module unlock being re-allocated and re-used for a lock inside of
a rcu-sched grace period. This is a very unlikely scenario, still
better plug the hole.

Use RCU list iteration in all places and ammend the comments.

Change lockdep_free_key_range() to issue a sync_sched() between
removal from the lists and returning -- which results in the memory
being freed. Further ensure the callers are placed correctly and
comment the requirements.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Tsyvarev <tsyvarev@ispras.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -633,7 +633,7 @@ static int count_matching_names(struct lock_class *new_class)
 	if (!new_class->name)
 		return 0;
 
-	list_for_each_entry(class, &all_lock_classes, lock_entry) {
+	list_for_each_entry_rcu(class, &all_lock_classes, lock_entry) {
 		if (new_class->key - new_class->subclass == class->key)
 			return class->name_version;
 		if (class->name && !strcmp(class->name, new_class->name))
@@ -700,10 +700,12 @@ look_up_lock_class(struct lockdep_map *lock, unsigned int subclass)
 	hash_head = classhashentry(key);
 
 	/*
-	 * We can walk the hash lockfree, because the hash only
-	 * grows, and we are careful when adding entries to the end:
+	 * We do an RCU walk of the hash, see lockdep_free_key_range().
 	 */
-	list_for_each_entry(class, hash_head, hash_entry) {
+	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
+		return NULL;
+
+	list_for_each_entry_rcu(class, hash_head, hash_entry) {
 		if (class->key == key) {
 			/*
 			 * Huh! same key, different name? Did someone trample
@@ -728,7 +730,8 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
 	struct lockdep_subclass_key *key;
 	struct list_head *hash_head;
 	struct lock_class *class;
-	unsigned long flags;
+
+	DEBUG_LOCKS_WARN_ON(!irqs_disabled());
 
 	class = look_up_lock_class(lock, subclass);
 	if (likely(class))
@@ -750,28 +753,26 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
 	key = lock->key->subkeys + subclass;
 	hash_head = classhashentry(key);
 
-	raw_local_irq_save(flags);
 	if (!graph_lock()) {
-		raw_local_irq_restore(flags);
 		return NULL;
 	}
 	/*
 	 * We have to do the hash-walk again, to avoid races
 	 * with another CPU:
 	 */
-	list_for_each_entry(class, hash_head, hash_entry)
+	list_for_each_entry_rcu(class, hash_head, hash_entry) {
 		if (class->key == key)
 			goto out_unlock_set;
+	}
+
 	/*
 	 * Allocate a new key from the static array, and add it to
 	 * the hash:
 	 */
 	if (nr_lock_classes >= MAX_LOCKDEP_KEYS) {
 		if (!debug_locks_off_graph_unlock()) {
-			raw_local_irq_restore(flags);
 			return NULL;
 		}
-		raw_local_irq_restore(flags);
 
 		print_lockdep_off("BUG: MAX_LOCKDEP_KEYS too low!");
 		dump_stack();
@@ -798,23 +799,19 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
 
 	if (verbose(class)) {
 		graph_unlock();
-		raw_local_irq_restore(flags);
 
 		printk("\nnew class %p: %s", class->key, class->name);
 		if (class->name_version > 1)
 			printk("#%d", class->name_version);
 		printk("\n");
 		dump_stack();
 
-		raw_local_irq_save(flags);
 		if (!graph_lock()) {
-			raw_local_irq_restore(flags);
 			return NULL;
 		}
 	}
 out_unlock_set:
 	graph_unlock();
-	raw_local_irq_restore(flags);
 
 out_set_class_cache:
 	if (!subclass || force)
@@ -870,11 +867,9 @@ static int add_lock_to_list(struct lock_class *class, struct lock_class *this,
 	entry->distance = distance;
 	entry->trace = *trace;
 	/*
-	 * Since we never remove from the dependency list, the list can
-	 * be walked lockless by other CPUs, it's only allocation
-	 * that must be protected by the spinlock. But this also means
-	 * we must make new entries visible only once writes to the
-	 * entry become visible - hence the RCU op:
+	 * Both allocation and removal are done under the graph lock; but
+	 * iteration is under RCU-sched; see look_up_lock_class() and
+	 * lockdep_free_key_range().
 	 */
 	list_add_tail_rcu(&entry->entry, head);
 
@@ -1025,7 +1020,9 @@ static int __bfs(struct lock_list *source_entry,
 		else
 			head = &lock->class->locks_before;
 
-		list_for_each_entry(entry, head, entry) {
+		DEBUG_LOCKS_WARN_ON(!irqs_disabled());
+
+		list_for_each_entry_rcu(entry, head, entry) {
 			if (!lock_accessed(entry)) {
 				unsigned int cq_depth;
 				mark_lock_accessed(entry, lock);
@@ -2022,7 +2019,7 @@ static inline int lookup_chain_cache(struct task_struct *curr,
 	 * We can walk it lock-free, because entries only get added
 	 * to the hash:
 	 */
-	list_for_each_entry(chain, hash_head, entry) {
+	list_for_each_entry_rcu(chain, hash_head, entry) {
 		if (chain->chain_key == chain_key) {
 cache_hit:
 			debug_atomic_inc(chain_lookup_hits);
@@ -2996,8 +2993,18 @@ void lockdep_init_map(struct lockdep_map *lock, const char *name,
 	if (unlikely(!debug_locks))
 		return;
 
-	if (subclass)
+	if (subclass) {
+		unsigned long flags;
+
+		if (DEBUG_LOCKS_WARN_ON(current->lockdep_recursion))
+			return;
+
+		raw_local_irq_save(flags);
+		current->lockdep_recursion = 1;
 		register_lock_class(lock, subclass, 1);
+		current->lockdep_recursion = 0;
+		raw_local_irq_restore(flags);
+	}
 }
 EXPORT_SYMBOL_GPL(lockdep_init_map);
 
@@ -3887,9 +3894,17 @@ static inline int within(const void *addr, void *start, unsigned long size)
 	return addr >= start && addr < start + size;
 }
 
+/*
+ * Used in module.c to remove lock classes from memory that is going to be
+ * freed; and possibly re-used by other modules.
+ *
+ * We will have had one sync_sched() before getting here, so we're guaranteed
+ * nobody will look up these exact classes -- they're properly dead but still
+ * allocated.
+ */
 void lockdep_free_key_range(void *start, unsigned long size)
 {
-	struct lock_class *class, *next;
+	struct lock_class *class;
 	struct list_head *head;
 	unsigned long flags;
 	int i;
@@ -3905,7 +3920,7 @@ void lockdep_free_key_range(void *start, unsigned long size)
 		head = classhash_table + i;
 		if (list_empty(head))
 			continue;
-		list_for_each_entry_safe(class, next, head, hash_entry) {
+		list_for_each_entry_rcu(class, head, hash_entry) {
 			if (within(class->key, start, size))
 				zap_class(class);
 			else if (within(class->name, start, size))
@@ -3916,11 +3931,25 @@ void lockdep_free_key_range(void *start, unsigned long size)
 	if (locked)
 		graph_unlock();
 	raw_local_irq_restore(flags);
+
+	/*
+	 * Wait for any possible iterators from look_up_lock_class() to pass
+	 * before continuing to free the memory they refer to.
+	 *
+	 * sync_sched() is sufficient because the read-side is IRQ disable.
+	 */
+	synchronize_sched();
+
+	/*
+	 * XXX at this point we could return the resources to the pool;
+	 * instead we leak them. We would need to change to bitmap allocators
+	 * instead of the linear allocators we have now.
+	 */
 }
 
 void lockdep_reset_lock(struct lockdep_map *lock)
 {
-	struct lock_class *class, *next;
+	struct lock_class *class;
 	struct list_head *head;
 	unsigned long flags;
 	int i, j;
@@ -3948,7 +3977,7 @@ void lockdep_reset_lock(struct lockdep_map *lock)
 		head = classhash_table + i;
 		if (list_empty(head))
 			continue;
-		list_for_each_entry_safe(class, next, head, hash_entry) {
+		list_for_each_entry_rcu(class, head, hash_entry) {
 			int match = 0;
 
 			for (j = 0; j < NR_LOCKDEP_CACHING_CLASSES; j++)

kernel/module.c

@@ -1865,7 +1865,7 @@ static void free_module(struct module *mod)
 	kfree(mod->args);
 	percpu_modfree(mod);
 
-	/* Free lock-classes: */
+	/* Free lock-classes; relies on the preceding sync_rcu(). */
 	lockdep_free_key_range(mod->module_core, mod->core_size);
 
 	/* Finally, free the core (containing the module structure) */
@@ -3349,9 +3349,6 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	module_bug_cleanup(mod);
 	mutex_unlock(&module_mutex);
 
-	/* Free lock-classes: */
-	lockdep_free_key_range(mod->module_core, mod->core_size);
-
 	/* we can't deallocate the module until we clear memory protection */
 	unset_module_init_ro_nx(mod);
 	unset_module_core_ro_nx(mod);
@@ -3375,6 +3372,9 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	synchronize_rcu();
 	mutex_unlock(&module_mutex);
  free_module:
+	/* Free lock-classes; relies on the preceding sync_rcu() */
+	lockdep_free_key_range(mod->module_core, mod->core_size);
+
 	module_deallocate(mod, info);
  free_copy:
 	free_copy(info);