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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