slab, lockdep: Annotate slab -> rcu -> debug_object -> slab

Lockdep thinks there's lock recursion through:

	kmem_cache_free()
	  cache_flusharray()
	    spin_lock(&l3->list_lock)  <----------------.
	    free_block()                                |
	      slab_destroy()                            |
		call_rcu()                              |
		  debug_object_activate()               |
		    debug_object_init()                 |
		      __debug_object_init()             |
			kmem_cache_alloc()              |
			  cache_alloc_refill()          |
			    spin_lock(&l3->list_lock) --'

Now debug objects doesn't use SLAB_DESTROY_BY_RCU and hence there is no
actual possibility of recursing. Luckily debug objects marks it slab
with SLAB_DEBUG_OBJECTS so we can identify the thing.

Mark all SLAB_DEBUG_OBJECTS (all one!) slab caches with a special
lockdep key so that lockdep sees its a different cachep.

Also add a WARN on trying to create a SLAB_DESTROY_BY_RCU |
SLAB_DEBUG_OBJECTS cache, to avoid possible future trouble.
Reported-and-tested-by: Sebastian Siewior <sebastian@breakpoint.cc>
[ fixes to the initial patch ]
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1311341165.27400.58.camel@twinsSigned-off-by: Ingo Molnar <mingo@elte.hu>

mm/slab.c

@@ -622,22 +622,22 @@ int slab_is_available(void)
 static struct lock_class_key on_slab_l3_key;
 static struct lock_class_key on_slab_alc_key;
 
-static void init_node_lock_keys(int q)
-{
-	struct cache_sizes *s = malloc_sizes;
+static struct lock_class_key debugobj_l3_key;
+static struct lock_class_key debugobj_alc_key;
 
-	if (g_cpucache_up != FULL)
-		return;
-
-	for (s = malloc_sizes; s->cs_size != ULONG_MAX; s++) {
+static void slab_set_lock_classes(struct kmem_cache *cachep,
+		struct lock_class_key *l3_key, struct lock_class_key *alc_key,
+		int q)
+{
 	struct array_cache **alc;
 	struct kmem_list3 *l3;
 	int r;
 
-		l3 = s->cs_cachep->nodelists[q];
-		if (!l3 || OFF_SLAB(s->cs_cachep))
-			continue;
-		lockdep_set_class(&l3->list_lock, &on_slab_l3_key);
+	l3 = cachep->nodelists[q];
+	if (!l3)
+		return;
+
+	lockdep_set_class(&l3->list_lock, l3_key);
 	alc = l3->alien;
 	/*
 	 * FIXME: This check for BAD_ALIEN_MAGIC
@@ -647,12 +647,42 @@ static void init_node_lock_keys(int q)
 	 * for alloc_alien_cache,
 	 */
 	if (!alc || (unsigned long)alc == BAD_ALIEN_MAGIC)
-			continue;
+		return;
 	for_each_node(r) {
 		if (alc[r])
-				lockdep_set_class(&alc[r]->lock,
-					&on_slab_alc_key);
+			lockdep_set_class(&alc[r]->lock, alc_key);
 	}
+}
+
+static void slab_set_debugobj_lock_classes_node(struct kmem_cache *cachep, int node)
+{
+	slab_set_lock_classes(cachep, &debugobj_l3_key, &debugobj_alc_key, node);
+}
+
+static void slab_set_debugobj_lock_classes(struct kmem_cache *cachep)
+{
+	int node;
+
+	for_each_online_node(node)
+		slab_set_debugobj_lock_classes_node(cachep, node);
+}
+
+static void init_node_lock_keys(int q)
+{
+	struct cache_sizes *s = malloc_sizes;
+
+	if (g_cpucache_up != FULL)
+		return;
+
+	for (s = malloc_sizes; s->cs_size != ULONG_MAX; s++) {
+		struct kmem_list3 *l3;
+
+		l3 = s->cs_cachep->nodelists[q];
+		if (!l3 || OFF_SLAB(s->cs_cachep))
+			continue;
+
+		slab_set_lock_classes(s->cs_cachep, &on_slab_l3_key,
+				&on_slab_alc_key, q);
 	}
 }
 
@@ -671,6 +701,14 @@ static void init_node_lock_keys(int q)
 static inline void init_lock_keys(void)
 {
 }
+
+static void slab_set_debugobj_lock_classes_node(struct kmem_cache *cachep, int node)
+{
+}
+
+static void slab_set_debugobj_lock_classes(struct kmem_cache *cachep)
+{
+}
 #endif
 
 /*
@@ -1264,6 +1302,8 @@ static int __cpuinit cpuup_prepare(long cpu)
 		spin_unlock_irq(&l3->list_lock);
 		kfree(shared);
 		free_alien_cache(alien);
+		if (cachep->flags & SLAB_DEBUG_OBJECTS)
+			slab_set_debugobj_lock_classes_node(cachep, node);
 	}
 	init_node_lock_keys(node);
 
@@ -2426,6 +2466,16 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 		goto oops;
 	}
 
+	if (flags & SLAB_DEBUG_OBJECTS) {
+		/*
+		 * Would deadlock through slab_destroy()->call_rcu()->
+		 * debug_object_activate()->kmem_cache_alloc().
+		 */
+		WARN_ON_ONCE(flags & SLAB_DESTROY_BY_RCU);
+
+		slab_set_debugobj_lock_classes(cachep);
+	}
+
 	/* cache setup completed, link it into the list */
 	list_add(&cachep->next, &cache_chain);
 oops: