debugobjects: fix lockdep warning

Daniel J. Blueman reported:
> =======================================================
> [ INFO: possible circular locking dependency detected ]
> 2.6.27-rc4-224c #1
> -------------------------------------------------------
> hald/4680 is trying to acquire lock:
>  (&n->list_lock){++..}, at: [<ffffffff802bfa26>] add_partial+0x26/0x80
>
> but task is already holding lock:
>  (&obj_hash[i].lock){++..}, at: [<ffffffff8041cfdc>]
> debug_object_free+0x5c/0x120

We fix it by moving the actual freeing to outside the lock (the lock
now only protects the list).

The pool lock is also promoted to irq-safe (suggested by Dan). It's
necessary because free_pool is now called outside the irq disabled
region. So we need to protect against an interrupt handler which calls
debug_object_init().

[tglx@linutronix.de: added hlist_move_list helper to avoid looping
		     through the list twice]

Reported-by: Daniel J Blueman <daniel.blueman@gmail.com>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 36 insertions, 8 deletions

diff --git a/include/linux/list.h b/include/linux/list.h
index db35ef02e745..969f6e92d089 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -619,6 +619,19 @@ static inline void hlist_add_after(struct hlist_node *n,
 		next->next->pprev  = &next->next;
 }
 
+/*
+ * Move a list from one list head to another. Fixup the pprev
+ * reference of the first entry if it exists.
+ */
+static inline void hlist_move_list(struct hlist_head *old,
+				   struct hlist_head *new)
+{
+	new->first = old->first;
+	if (new->first)
+		new->first->pprev = &new->first;
+	old->first = NULL;
+}
+
 #define hlist_entry(ptr, type, member) container_of(ptr,type,member)
 
 #define hlist_for_each(pos, head) \
diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 45a6bde762d1..e3ab374e1334 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -112,6 +112,7 @@ static struct debug_obj *lookup_object(void *addr, struct debug_bucket *b)
 
 /*
  * Allocate a new object. If the pool is empty, switch off the debugger.
+ * Must be called with interrupts disabled.
  */
 static struct debug_obj *
 alloc_object(void *addr, struct debug_bucket *b, struct debug_obj_descr *descr)
@@ -148,17 +149,18 @@ alloc_object(void *addr, struct debug_bucket *b, struct debug_obj_descr *descr)
 static void free_object(struct debug_obj *obj)
 {
 	unsigned long idx = (unsigned long)(obj - obj_static_pool);
+	unsigned long flags;
 
 	if (obj_pool_free < ODEBUG_POOL_SIZE || idx < ODEBUG_POOL_SIZE) {
-		spin_lock(&pool_lock);
+		spin_lock_irqsave(&pool_lock, flags);
 		hlist_add_head(&obj->node, &obj_pool);
 		obj_pool_free++;
 		obj_pool_used--;
-		spin_unlock(&pool_lock);
+		spin_unlock_irqrestore(&pool_lock, flags);
 	} else {
-		spin_lock(&pool_lock);
+		spin_lock_irqsave(&pool_lock, flags);
 		obj_pool_used--;
-		spin_unlock(&pool_lock);
+		spin_unlock_irqrestore(&pool_lock, flags);
 		kmem_cache_free(obj_cache, obj);
 	}
 }
@@ -171,6 +173,7 @@ static void debug_objects_oom(void)
 {
 	struct debug_bucket *db = obj_hash;
 	struct hlist_node *node, *tmp;
+	HLIST_HEAD(freelist);
 	struct debug_obj *obj;
 	unsigned long flags;
 	int i;
@@ -179,11 +182,14 @@ static void debug_objects_oom(void)
 
 	for (i = 0; i < ODEBUG_HASH_SIZE; i++, db++) {
 		spin_lock_irqsave(&db->lock, flags);
-		hlist_for_each_entry_safe(obj, node, tmp, &db->list, node) {
+		hlist_move_list(&db->list, &freelist);
+		spin_unlock_irqrestore(&db->lock, flags);
+
+		/* Now free them */
+		hlist_for_each_entry_safe(obj, node, tmp, &freelist, node) {
 			hlist_del(&obj->node);
 			free_object(obj);
 		}
-		spin_unlock_irqrestore(&db->lock, flags);
 	}
 }
 
@@ -498,8 +504,9 @@ void debug_object_free(void *addr, struct debug_obj_descr *descr)
 		return;
 	default:
 		hlist_del(&obj->node);
+		spin_unlock_irqrestore(&db->lock, flags);
 		free_object(obj);
-		break;
+		return;
 	}
 out_unlock:
 	spin_unlock_irqrestore(&db->lock, flags);
@@ -510,6 +517,7 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size)
 {
 	unsigned long flags, oaddr, saddr, eaddr, paddr, chunks;
 	struct hlist_node *node, *tmp;
+	HLIST_HEAD(freelist);
 	struct debug_obj_descr *descr;
 	enum debug_obj_state state;
 	struct debug_bucket *db;
@@ -545,11 +553,18 @@ repeat:
 				goto repeat;
 			default:
 				hlist_del(&obj->node);
-				free_object(obj);
+				hlist_add_head(&obj->node, &freelist);
 				break;
 			}
 		}
 		spin_unlock_irqrestore(&db->lock, flags);
+
+		/* Now free them */
+		hlist_for_each_entry_safe(obj, node, tmp, &freelist, node) {
+			hlist_del(&obj->node);
+			free_object(obj);
+		}
+
 		if (cnt > debug_objects_maxchain)
 			debug_objects_maxchain = cnt;
 	}