Re: slub/debugobjects: lockup when freeing memory

[Paul E. McKenney]

On Tue, Aug 19, 2014 at 09:00:05PM -0500, Christoph Lameter wrote:
> 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.

That is correct -- the debug-objects code uses separate storage to track
states, and does not touch the memory to which the state applies.

> 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.

And indeed they are, good point!  It appears to me that both sets of
#ifdefs can go away.

							Thanx, Paul

> 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);
>  }
> 

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