Re: slub/debugobjects: lockup when freeing memory

[Christoph Lameter <cl@linux.com>]

On Mon, 18 Aug 2014, Paul E. McKenney wrote:

> > +#ifdef CONFIG_RCU_DEBUG_XYZ
>
> If you make CONFIG_RCU_DEBUG_XYZ instead be CONFIG_DEBUG_OBJECTS_RCU_HEAD,
> then it will automatically show up when it needs to.

Ok.

> The rest looks plausible, for whatever that is worth.

We talked in the hallway about init_rcu_head not touching
the contents of the rcu_head. If that is the case then we can simplify
the patch.

We could also remove the #ifdefs if init_rcu_head and destroy_rcu_head
are no ops if CONFIG_DEBUG_RCU_HEAD is not defined.



Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c
+++ linux/mm/slub.c
@@ -1308,6 +1308,25 @@ static inline struct page *alloc_slab_pa
 	return page;
 }

+#define need_reserve_slab_rcu						\
+	(sizeof(((struct page *)NULL)->lru) < sizeof(struct rcu_head))
+
+static struct rcu_head *get_rcu_head(struct kmem_cache *s, struct page *page)
+{
+	if (need_reserve_slab_rcu) {
+		int order = compound_order(page);
+		int offset = (PAGE_SIZE << order) - s->reserved;
+
+		VM_BUG_ON(s->reserved != sizeof(struct rcu_head));
+		return page_address(page) + offset;
+	} else {
+		/*
+		 * RCU free overloads the RCU head over the LRU
+		 */
+		return (void *)&page->lru;
+	}
+}
+
 static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 {
 	struct page *page;
@@ -1357,6 +1376,22 @@ static struct page *allocate_slab(struct
 			kmemcheck_mark_unallocated_pages(page, pages);
 	}

+#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
+	if (unlikely(s->flags & SLAB_DESTROY_BY_RCU) && page)
+		/*
+		 * Initialize various things. However, this init is
+	 	 * not allowed to modify the contents of the rcu head.
+		 * Allocations are permitted. However, the use of
+		 * the same cache or another cache with SLAB_RCU_DESTROY
+		 * set may cause additional recursions.
+		 *
+		 * So in order to be safe the slab caches used
+		 * in init_rcu_head should be restricted to be of the
+		 * non rcu kind only.
+		 */
+		init_rcu_head(get_rcu_head(s, page));
+#endif
+
 	if (flags & __GFP_WAIT)
 		local_irq_disable();
 	if (!page)
@@ -1452,13 +1487,13 @@ static void __free_slab(struct kmem_cach
 	memcg_uncharge_slab(s, order);
 }

-#define need_reserve_slab_rcu						\
-	(sizeof(((struct page *)NULL)->lru) < sizeof(struct rcu_head))
-
 static void rcu_free_slab(struct rcu_head *h)
 {
 	struct page *page;

+#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
+	destroy_rcu_head(h);
+#endif
 	if (need_reserve_slab_rcu)
 		page = virt_to_head_page(h);
 	else
@@ -1469,24 +1504,9 @@ static void rcu_free_slab(struct rcu_hea

 static void free_slab(struct kmem_cache *s, struct page *page)
 {
-	if (unlikely(s->flags & SLAB_DESTROY_BY_RCU)) {
-		struct rcu_head *head;
-
-		if (need_reserve_slab_rcu) {
-			int order = compound_order(page);
-			int offset = (PAGE_SIZE << order) - s->reserved;
-
-			VM_BUG_ON(s->reserved != sizeof(*head));
-			head = page_address(page) + offset;
-		} else {
-			/*
-			 * RCU free overloads the RCU head over the LRU
-			 */
-			head = (void *)&page->lru;
-		}
-
-		call_rcu(head, rcu_free_slab);
-	} else
+	if (unlikely(s->flags & SLAB_DESTROY_BY_RCU))
+		call_rcu(get_rcu_head(s, page), rcu_free_slab);
+	else
 		__free_slab(s, page);
 }